Nice suggestions, thanks!

Cardboard by itself isn't entirely trustworthy (all bets are off should I fall on the blade at an unlucky angle). But reinforced with other materials as suggested, it should be a considerably more robust.

The polypropylene cutting mat is an option I hadn't thought of. Nice touch using the hotel hair dryer to shape it! I imagine very hot water might work also.

The leather belt is a good approach, either by itself or combined with tape and other materials. Better still: thrift shops often receive belts that aren't good enough to resell, and if you ask in the back you can have them for free.

One thing I have used successfully is 1/2" PEX tubing (cross-linked polypropylene) used for water lines in new construction. There are often discarded chunks at construction sites. This stuff is super tough to cut (hacksaw preferred) but if you can make a slit, it's good protection for any edge including machetes and axes. I imagine you could melt a slit into it with a nail and a heat source.