Best place to put sleeping bag for noob? Water filter?

Inside would be better, straps are never tight enough to eliminate sway and having parts of the load shifting as you walk is both annoying and, in rougher ground, potentially dangerous as it could cause a fall as the load shifts and you lose your balance at just the wrong moment.

most any down bag I can think of would fit in a 70L pack sleeping bag compartment. What pack and what sleeping bag?

Plus just try stuffing it directly into the compartment without it being in its stuff sack. That might allow it to conform better to the compartments shape, which isn't cylindrical so not a good match to the stuff sack. Also those compartments are often adjustable: try loosening the inner adjustment straps. (If any).

I agree with almostthere--if you can't get a sleeping bag in a 70L pack, something's not right. Either the bag is not made for backpacking or you're carrying way too much other stuff.

I met a guy on the trail once who had tied his bag on the outside of his pack. He told his story: the ties snagged on a snag and the bag went tumbling down a steep slope into a very brushy area. He then made two big mistakes (three if you count putting the bag on the outside as #1). First, instead of dropping his pack on the trail, he headed down the slope with his pack on to find his bag. Second, when the pack tired him out, he dropped it on the slope and continued down. He never found the bag, though he looked until it got dark. Then he couldn't find where he'd dropped his pack. He spent the night in the brush on that slope; said he didn't think he was going to survive it. The "d'oh!" moral of the story: Things tied on the outside of a pack can fall off.

Besides falling off, they provide an opportunity for trees and bushes to grab your pack as you pass by.   I profoundly dislike feeling that horrible tug from behind about three steps beyond the tree.

If I had all the right gear in the beginning I would have never started backpacking. Without knowing anything about backpacking other than what was available on the net in '96 I purchased my first pack. It was Kelty Arapaho 6400cc (104L). Way way too big but I had no idea. Over the next few years I bought some gear here and there slowly. Still without knowing much. 10 years later when I finally went backpacking for the first time if it hadn't been for my gigantic pack I never would have went because resources were tight and in no way could I have replaced much if anything.

My first sleeping bag was synthetic 20 degree SlumberJack that probably weighs 6lbs or more. It rolled into the size of probably a 40l pack but it fit in the sleeping bag compartment in the Arapaho. I learned a lot on my very first hike which was 4 days/3 nights. Over the next 10 years I started slowly replacing things. First thing was my sleeping bag. I got a sweet deal on a Blue Kazoo. As my gear slowly got smaller and lighter I finally got to the point that my pack was just too big and needed to be replaced. I ended up with 65l which is what I still use today.

Yes, a sleeping bag strapped to the outside is probably not the best choice. But this isn't winter and his/her first hike probably isn't more than 1 or 2 nights. Let's just encourage him to get out there and do it. Make adjustments after the fact. Put a garbage bag around it, strap it very securely to the outside and go have fun. Come back and tell us what you've learned and how you'll do things differently.

Having the experience of shepherding many noobs out there, I believe we do none of them any favors by not telling the truth. If the bag fills a 70 liter pack it's a car camping bag. Especially if the noob needs a bear canister and has other heavy gear -- any noob can do better than a 70 liter sleeping bag.

I have a five pound synthetic bag - AND IT DOES NOT TAKE UP A 70 LITER PACK. Not an unrealistic expectation to find something smaller than that!

Nope: that's a setup for a more uncomfortable trip than is possibly necessary. Happy face cheerleading to an experience that could have been avoided doesn't do anyone, new or not, any favors. Especially when they actually asked for advice.

Sure suggesting custom backpacks and thousands of dollars in boutique gear would be rude and disrespectful. But no one has done that. (In this thread anyway...)

That *oh he'll survive, we did* approach neglects the point most new people are thinking of going backpacking for fun. "Surviving" is not usually fun. No fun: no going out again. I've known plenty of people who went one and done. Almost always related to avoidable errors: but the first impression is all it takes.

almosthere: the mention of a "case" might mean that 3lb bag was just provided with an overly generous stuff sack. In which case either my suggestion of just stuffing it in the compartment or a compression stuff sack (the OP did ask for buying solutions) could solve the issue. I'd be a little envious frankly. Every sleeping bag I've ever bought came in a sack that it close to NEVER went backinto without thus huge fight.

Here's a range of compression stuff sacks to look at.
www.rei.com/product/730882/sea-to-summit-event-compression-dry-sack

I've actually seen a near-70 liter sleeping bag. We took a guy on his first backpack in 50 years (we didn't know that until he told us on the hike back to the car) on one of the meetup hikes. He had a dinosaur of an internal frame that was nearly 90 liters big, he himself was nearly seven feet tall, he packed for a three day weekend of about 12 miles total a massive bear can full of food, this very old synthetic bag (he swore it was down, it wasn't), three bath towels, about seven shirts, a couple pairs of pants, two "survival" kits of dubious origin, a five pound book on Python (the programming language), several pairs of socks, basically whatever he thought sounded good.

On the way in, he already had people carrying some of his stuff -- the DSLR, the set of stainless steel pots, the random junk he had floating around in the top of the pack. On the way out his knee (carefully swaddled in the knee brace he wore the whole time) really bothered him and so he sat down and declared himself done just a mile from the parking lot. We walked out, dumped the contents of our packs in the cars, returned, and my 65 liter pack almost managed to contain just his sleeping bag, which granted was made for Mr. Big and Tall, but seriously it probably weighed 15 lbs all by itself. He hadn't eaten any of the food -- the Mountain House was too spicy for his upset tummy, he existed on my bland instant potatoes that I gave to him when he couldn't manage eating chili mac -- and he hadn't read the book or used the towels, or the shirts. He hadn't slept at all -- we all heard him tossing and turning and moaning in his three person tent all night for two nights.

Coupe de grace -- he confessed as we walked his stuff out that his doctor had instructed him to never carry more than 15 pounds on his back. He had almost 75 lbs, we figured, but were too depressed and tired to actually weigh it out.

No matter how many mistakes you make as a noob, you are not That Guy. Even if you have an eight pound sleeping bag.