Do they do the smaller one at the bottom of the rear wheel arch? It clips the the sill trim (Mk3) to the arch lip. I removed mine for cleaning and undersealing but it got a bit mangled. I used another size but had to shave it down to fit.
Can do better than that, below is a pic of the actual clip, the good one from the other side.
To compare I had a trim clip, the one they use to hold the plastic boot lining trim in place. The clip I need is on the right, measures 11mm dia and the shank part 20mm
As said it holds the rear part of the sill trim to the bottom of the wheel arch.
I tried them on mine. It wasnt tight, so I’m going to try a different one.
I did just about get one of those larger ones to fit, but had to skim a bit off it to get it in. Ooo er.
I can probably see your confusion with mentioning trim clips and boot trim clips etc.
If you look at a Mk3 rear wheel arch, on the outside bodywork on the lower part of the arch lip there is a plastic stud/clip, this clips the end of the outer plastic sill covering to the arch lip. It’s smaller than the rest of the studs/clips on the car, see pic in post above.
It’s only a plastic clip but it seems the usual suppliers of mx5 parts don’t have them, although I’ve not tried the dealers yet.