I like the wing / spoiler idea. An assembly of parts for a three piece wing, with compound curves, straight lines, beveled edges and maybe incorporate a center third brakelight for additional componentry. You can demonstrate / indicate the positive or negative slope to the wing by showing the degrees-of-angle chart off to the side (since its a 2D drawing), hardware assembly to the decklid, and wiring ports for the brakelight.
Easy as pie!
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I USED TO HAVE an 85 Camaro 305 4bbl, 90 Firebird V6, 97 Firebird V6 (10 speakers!). Despite the two family vehicles, 3 kids, wife and mortgage, I can finally realize the dream - 88 TA almost all stock... for now.
http://www.cardomain.com/id/untameddave
Wanted this -

, but ended up with this -

for free. Can't argue with free.