Foam rubber...like a big sponge or mattress foam block. Cut it so it fits in deep...it will probably take 2 big blocks. Kinda force fit it in there. As it gits cold....it will shrink up some so don't make it loose.
What about making the block from cloroplast of cut it out of styrofoam SM and covering the grill block with vinyl upholstery. Attach it with black plastic ties. The upholstery will prevent scratching. I haven't had any scratching with mine anyway.