Great job "improvising".
I've made some quick sheaths out of corrogated cardboard over the years. They ain't pretty, but they do the job.
Just take a section of thick corrogated cardboard from a box. See which way the inside layers run. Then slowly push your knife blade in between the outside layers. The inside layers will slowly cut or push out of the way. Now take your knife back out and lay it on top of that cardboard. Trace around it, then mark another line around one inch farther out. Then cut out that new temp "sheath". I just cut it out square - leaving plenty of extra around the blade.
These are very temporary, but also very functional. They also are far less "safe" than a true sheath, but sure beat having that uncovered blade floating around in your bag.
I've also made quick sheaths by just folding over a piece of that cardboard over the blade, and then stapling the two parts together just outside the blade. Hey, I'll even admit to folding a piece of cardboard around a knife blade, and then wrapping it all in duct tape! It's quick, and it works. Anything to keep from knicking my fingers when digging that knife out of a bag.
Just some thoughts to share.
Mikey - yee ol' grumpy blacksmith out in the Hinterlands