[kaisen]

I'm with Byron on this one.

Buy a used center row bucket seat (I assume you have two buckets in the center row). Visit car-part.com and look for them in your area. Around here they are $100-150 each.

You can keep the base mechanism and remove the seat cushion and entire backrest. Then fabricate a way to bolt an appropriately sized racing seat with harness upon the base. When you're not using it, it should retain removeability so you can swap in your old seat, or remove them for cargo.