we used 1/4 inch high density foam i think is whats its called. you can get it from interior shops. you just cut out the design you want and glue it down then just put the material over the whole thing it the design will stick up. only difference in doing this then a regular panel is that you have to tuck the material around the design you do, its pretty simple.
if you want to just do outlines of flames and stuff,you can use yarn string,spray a contact cement,or any kind of interior glue,let it tack up a bit(sticky to the touch),then take your yarn and draw out whatever you want
All as said above is great, 1/4" foam, and 1/8 foam. I have used flat aluminum and abs panels for things before too. (larger designs, like in truck beds, etc.) DAP weldwood is the glue to use.