Before going to a driving school at Willow Springs I decided to try to get more clearance between my head and my Hard Dog Hard Core rollbar by removing some foam from the driver's seat. Also, I thought that removing foam would make the seat more supportive.
I have Wet Okole neoprene seat covers (#01) which does add about 1/8" of padding to the seats. But I like the looks of it so I keep it on the car.
Seat removal is pretty easy, just make sure before pulling the seat out that the headrest speaker cable and some other cable needs are unplugged (#02, #03).
Once the seat is removed, I began removing the hog rings (#04) that keep the OEM seat cover in place. My goal was to remove the entire lower cushion from the frame because I saw it what another Miata owner had done. But I didn't realize that there were a bunch of hog rings (#06) positioned on both left and right "channels" of the seat foam, as well as the back part of the cushion.
Once the seat cushion is removed (#08), the next decision is whether to cut foam from the bottom of the cushion or scoop out foam from the top of the cushion. I decided to scoop it out from the top. The job was made much easier with an electric knife (#12) which cut the foam like butter, in nice wide swaths. I can just imagine that cutting the foam manualy with a utility or other kind of knife would make the job much more difficult.
Once I got enough foam scooped out, I put the OEM seat cover back together again. I took the easy way out and decided to use nylon tie wraps (#13) instead of hog rings. I think I probably ended up with 3/4" to 1" of extra headroom. Still not enough to have a 2" clearance below the rollbar with my helmet on, but at least now, the seat felt much more supportive.