My driver's seat foam was worn and its cover needed sewing so I decided to try repairing it (first time I ever modded a car seat).
I put a foam pad on the bottom (from a playroom floor tile),
and a same one (white this time) on top:
The cover was sewed by a specialist but it was a PAIN IN THE ASS to put it back.
Main problem was that the foam was worn and couldn't hold the mounting rods of the cover. I had to repair some spots with silicone:
I used zip ties instead of hog rings. Almost every upholsterer (original included) does the opposite but I prefer the fully adjustable nature of zip ties even though I just went full tight everywhere:
Not 100% wrinkleless because the foam can't hold the cover's mounts as it used to do but it is WAY much comfier with the pads I added. Zero cost, one day job, makes me and my butt happy :) Oh, I gave 5 euros to the guy for sewing though he barely asked any money.