I think my GTM is asking for a rear spoiler. That is going to be "Plan A" for me. "Plan B" will be a wing, only if "Plan A" fails.
My primary goal is function. I like the idea of low-drag downforce that is easily adjustable. If I buy a wing, APR will be the baseline and work up from there. I don't want anything folding in half at 150+ mph.
The biggest issue for fabricating a spoiler for the GTM is the shape of the rear deck, it would be much easier if it was a flat surface across the upper rear bodywork. I don't mind fabricating a subframe to support the stress on the bodywork.
My ideal solution is an adjustable "wickerbill" type spoiler that can be raised or lowered for different driving situations.

I especially like the C7 Z06 adjustable wickerbill concept.

stage3_z06_z07_rear_spoiler_painted.jpg

Here's a similar idea that I found from a racing simulator.

0Q5DIO9.jpg

I know tucker298 was working on a spoiler. I really like his design so far, I don't know how far he has gotten on it.
This flat cardboard mockup is very close to what I'm looking to do:
IMG_1112.jpg

I would love to hear any tips on making something like these work on the GTM rear. Thanks!