I'm not sure why the OP isn't considering USB external hard drives for backup purposes, because HDDs have a much better lifespan than the flash that you can find in a USB pen drive.
Probably the price. He is trying to find the cheapest possible solution. The problem is that if you want to save money on cheap hardware, you'll pay for it elsewhere.
Well, the OP can still makes a tails OS USB and create a cold stoage on a tails USB and if he maanages to set it up correctly, then it would be cheaper solution and reliable one too.
Details can be found here
How to Install Tails OS on USB flash drive for Wallet PurposeHowever, as the OP is thinking of having a complete blockchain on his pen drive or storing his wallet directly on a USB drive, such a solution doesn't exist and/or isn't feasible/secure.