Amazon has Black ABS sheets in a variety
of sizes, thicknesses, and textures.
ABS is rigid enough to hold it's own shape
yet somewhat flexible for ease of fitting.
It can also be formed into shape using heat.
Here's a 1 foot square piece, 1/16 inch thick
for $ 6.37 :
https://www.amazon.com/Install-Bay-ABS116-Universal-16-Inch/dp/B0049MWXM8/ref...For that price it might be worth an experiment.
I haven't tried it myself but it sounds promising.