Make one from the cheapest plywood you can find these days (maybe osb sheathing?) with 2x4 framing to hold it together, since the purpose would be to see if you fit the book seat.
The traditional method provides the most room and lowest seating position in the car:
A squab of plywood base, two inches of high density foam (not standard density upholstery foam), wrapped with vinyl stapled underneath, then velcro to the floor. Same for the back except one inch, same density foam. If you don't like it, pull it out. The most expensive part is new foam. Look for a product that you can use the foam from, like a yoga mat, sleeping bag pad, knee board, etc.
If you want more professional looking results but are satisfied with the squabs, take them to an upholstery shop to have the foam corners radiused (or you can do that yourself with very light application of a sanding disc) to have a cover with sewn corners.
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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS
viewtopic.php?f=5&t=13225&p=134742Simple Spring select
viewtopic.php?f=5&t=11815LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12