How about a coffin with an skeleton arm busting through the lid clinging to a Firebird Emblem?

Actually I like what you're discibing, pretty straight forward for a car show, we have a banner/sign company near me that makes about anything reasonbly priced as long as he can download logos and not have to draw it. Legal as long as you're not selling them. Carpet can get pricey, and personally I never liked the look since it's not natural for a car to be on carpet. The diamond flooring is ok, but it can look cheap.

Have lots of stuff and options with you, look at other displays to see if you have to much or not enough out.

Outside the box?
A beach display of your car with fake sand dunes behind it or dune grass or potted palms in the background, maybe inflateable beach stuff around it, gives it a parked at the beach look, may be a little unorthodoxed for a show and it might make judging tough, but it'd look good with a Convertible.