Lowes has rather nice/affordable closed-cell foam mats you can use for ALL sorts of projects: https://tinyurl.com/ych893zc
It's $20 for four 2'x2' ~1/2" thick interlocking sheets. It's easy to cut with a tile knife/box-cutter and seals up nicely with a heat gun for painting. One side has a faux grill-plate texture and the other side is smooth.