You talking about the ones like Summit racing sells? You buy the cover separate? You can cut the top or head rest off them and fiber glass them over. Cut the cover and sew them back up and make a gasser seat.
I wasn't sure which I have and which double wall seats you referred to in the beginning. I do have a set from summit racing in my coupe. The old set I took out of the coupe are heading to the MG. They move a little but not enough to be any problem. I have to hang on my roll bar and hoist myself into my seat over the side bar swing out section to get in and out and drop myself into the seats. I think they are pretty sturdy considering my size and weight. The ones for the MG are Jaz seats and the coupe are summit brand. The Jaz seats survived my accident but cracked in the seat area on my driver side. My sons side was fine. Guess that's why I had so many back issues after the accident. Hope this was helpful.
If you aren't going to put the car/truck on the track, the poly seats are really pretty comfy, even without any padding, the molded shape is terrific as-is.
They won't pass tech ANYWHERE though without a tube frame brace (like a mini roll bar tucked in all round the edges), solid floor mount, and a seat mount (usually 1/8" thick plate and tube) from the mid-back section to the roll bar.