How would you make a quick sheath for a fixed blade using only scrounged materials when you're far from home?

The scenario: I'm travelling by air, carry-on only. I get an unexpected invitation to see a really neat trail in a wilderness area, or go on the water for some fishing/crabbing. I have some airline-acceptable gear to take along, but no sharps, and that's uncomfortable. (This scenario has happened several times.)

So, I head to the local thrift shop and grab the best I can get, often a butcher, chef, or boning knife for a couple of bucks. It's not sexy, but I can put a coarse edge on it PDQ, and since I've practiced with these I know what it can do, and what its limits are.

After I'm done, I'll just donate it back to the same shop. Easy as pie.

But I need a way to protect the blade (and myself) while I carry it. Probably in a fanny pack etc. rather than on my belt.

Ideas?