cheap usb ("thumb") drives

The USB protocol only identifies 'mass storage device' characteristics, not read/write permissions. Any permission mechanism isn't standard (might work only on one filesystem type, or one vendor's mark 3 control model...).

You'd be better off using SD card (maybe plugging an SDcard micro module into a $0.79 USB reader), because the SD card specs DO include write locking by a number of mechanisms, some hardware and some software. Also, naturally, you'd want a checksum or hash that can be checked against the contents of any pretender to legitimacy...

If they have a full size sD slot then I favour Phils suggestion of a micro sD in an sD carrier write protected with a spec of araldite.

My favourite portable for travelling is an ASUS T1000 (not made of liquid metal) 11" diagonal screen. It's not fast, a bit long in the tooth now but lasts a long time and is very small. It even comes apart so that you can use the screen as a tablet.

That sounds like hard work for little gain. BIOSes can also be overwritten.

That's weak, though. The 'write protect' of the carrier is just a suggestion, not hardware-implemented, but sets a bit that can be polled. Or, ignored. The SD specification does include non-reversible write disable commands (and the SD card control logic will never enable the voltage boost necessary to write new data into the flash). That, IS a hardware write protection.

Check out the Wikipedia on SD card, there's references...