Folks have made fun of me for this, but I use Bish's Original Tear Mender Fabric & Leather Adhesive. You can get 6oz. for about $10 and I always have to throw the bottle out every couple of years - it lasts a while but forms a skin inside the bottle eventually.
This is an organic glue that bonds in 30 seconds with just hand pressure then dries enough to perform in an hour. Washes off everything but fabric similar to rubber cement - you can rub it off without water. Be sure that you air dry your fabric and not stuff it in your sack waiting to dry. If you aren't careful, it can be kinda unsightly - it bleeds through denim and dries white, for example. I use it to patch the crotch of my jeans & canvas shorts... I cut a piece of same/similar material and just glue it on like I'm 5 years old with a bottle of Elmers. Because it doesn't dry clear, I don't use it for thin fabrics like shirts... and if you're going to do a heavy-duty glue up on a fabric pack or canvas, I would let it dry for 24 hours before expecting a full strength bond.
They make smaller bottles too that would be easy to stuff in a pack. I'm not trying to convince anyone this stuff is better than sewing, but I use it because my sewing game is weak.