First of all, welcome! Looks like you've done some homework.
If you can take the spoiler off easily, just do some coast down testing and see if it shows up in the data. If not, measure 15-20* in from the flow detachment points (roof, pillar) to see if it seems to help create a phantom kammback. Some photos of your current setup would help as well, but you may have to post a few times before the site will let you.
I'd recommend building a Kammback out of metal bar and coroplast. It would end up looking like a CRX in side view and would fill in some of that wake. Cooling shouldn't be an issue if you leave some holes in the rear surface for [cooling] exhaust air.
PS- for others' reference, this seems to be a first generation "W10" MR2. Second gens debuted in 1989 so correct me if I'm wrong