Sounds ridiculous but it's actually pretty easy. It takes some time and uses about one roll of duct tape.

The model would be like that of the improvised courier bag, but silver with a wavy, uneven look to it.

Storage space ought to be around that of a smersh bag, 2x4... in fact, maybe 2x3

I think it would make it easier to find a bag as a fresh spawn and give that duct tape some purpose, too.

I would add a time consuming animation loop, however, since it really takes a while to make one in real life.