I have a Joutsen Tunturisusi 2000 sleeping bag, which of course came with the appropriate compression bag for storage. On a winter hike, I once repurposed that bag as a small backpack for a day's skiing trip out of camp. That served its purpose, but was rather uncomfortable to carry and overall quite inconvenient to use, as it was jerry rigged on the spur of the moment. So I thought, why not make a proper compression bag / backpack combination?
Yes, I probably overthought and overdesigned this thing, but that's what happens.
The original compression bag is fairly standard, and sized according to
the sleeping bag. Its design is what you'd expect. When the sleeping bag
isn't inside (i.e. it's left in camp or at a wilderness hut or
wherever), the sack is just big enough to accommodate a Trangia
stove or a pakki (mess kit), a fuel bottle, lunch supplies and
various odds and ends.
To make it more comfortable to carry as a backpack, I wanted to make room inside for a foam seat pad (that rainbow colored thing, which is also useful for sitting) which would also act as a back panel. Thus I had to make my new sack some 5 cm bigger in circumference (as determined by the yellow tape measure in the photo). I also made it some 5 cm taller. Not only do these give more room when using it as a backpack, but the sleeping bag can also be stored in a slightly less compressed state at home. With all the compression straps, I can easily compress it to fit inside my actual backpack without any problems. And as an added bonus, this sack won't suddenly be too tight in case I buy a new, slightly fluffier sleeping bag (not that I'm likely to wear out my current Tunturisusi 2000 any time soon). It is now quite oversized for my QTM 900 summer sleeping bag, however, but I've only ever needed this kind of day pack on winter trips. I rarely stay put in the same camp on a summer hike.
I made the sack out of waterproofed 330 denier Cordura
fabric—needlessly strong stuff, but hey. I sewed the seams with
Nybond bonded Nylon thread—again needlessly strong stuff.
Just like the original, the sack is tubular, with one end that can be
pinched shut with a sewn-in cord and locked with a cord stopper.
There are four lengthwise compression straps evenly spaced along the circumference of the sack, just like there are in the original. Two straps (at the sides, when viewed as a backpack) are sewn to the sack at their bottom ends. When released from their tension locks in the top lid, these straps double as the lower halves of the shoulder straps for the backpack. The other two compression straps are actually one long strap, sewn to the top lid, but free to slide all the way around underneath the sack, with a single buckle near the top end. That buckle is on the outward-facing side of the backpack, and serves to tighten the whole strap. That way I don't need a separate tension lock for the strap that's against my back—which would probably press painfully against my spine or neck.
The open end of this sack is covered by a top lid with two tension locks
on the sides. The compression straps on the sides attach to these.
When these side straps are loosened, and the buckle of the remaining
strap is released, the top lid can be opened to access the sack's
contents. There is a zipper pocket on the underside of the lid for small
items, as well as a larger zipper pocket inside the sack.
Three horizontal straps go around the whole sack. On the outward-facing side of the backpack, they are sewn to the sack following the Molle/PALS standard spacing, offering a place to attach an extra pocket or two if extra space is needed. The lengthwise compression straps can also be threaded through these Molle/PALS loops to help them stay in place. On the side facing my back, the horizontal straps are not sewn to the sack, and can thus be used as horizontal compression straps when the sack is used as a compression bag.
The upper parts of the shoulder straps, made from 50 mm webbing
(or similar seatbelt-like material salvaged from an old backpack), are
sewn close together near the top of the sack. Their free ends sport
25 mm tension locks. These straps are unnecessary when the sack
is used to store the sleeping bag, but they can be tucked away underneath
the top lid.
When the two side compression straps are released from their tension locks in the top lid, they can be threaded into the tension locks at the ends of the shoulder straps. This instantly turns the compression bag into a backpack!
Especially when a seat pad is against the back, the backpack is quite
comfortable. Quite small, though, but big enough for use as a
day pack. Plus there is the expansion option with Molle/PALS
pockets, which my main backpack always has attached to it.
With the side compression straps serving as the lower portions of the shoulder straps, the top lid was quite free to roll off to either side, so as a final finishing touch (not seen in the photo, as I had not done it yet), I sewed short straps on the sides near the top end of the sack. These can be threaded into the lid's side tension locks to keep it nicely in place.
I guess one could just sew shoulder straps onto an existing compression bag, but I said I overthought this. :)