Thanks for the suggestions, all!
In my exhausted, work-addled state last night, I decided to be lazy, and cut a small piece of cardboard from a box top. Rounded off the corners with a sanding stick, cut a slit for the shoulder belt pass-through, and painted it up. Overall it does the job - just about the right amount of depth, decent texture. I'd post a pic but the ones I took came out terribly. Will make another attempt tonight.
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com