What would be the best thickness and how would you fix it in the car. I know some is self adhesive so can make a ally sheet to fit in the bottom of the car but the rear panel is an inch too far back for comfy clutch operation? I'd need some form of frame to support the foam and ally sheet to take my weight.

What you could do is use the builders type foam in a can .

strong poly bag, fill a little sit on it , then fill the bag with the rest of the can,use another can if necessary
keep sitting till set or stops expanding

trim excess and that's a "hug your bum" fitted seat base,,upholster as required

bigger the can the better, try builder merchant and don't say what it's for cos they'll just have a laugh