I've made seat pans out of plywood before not to save $ just wanted a different seat, that foam is expensive no matter where you get it, didn't save $ there.
The Joanne shop for the vinyl, some of it could be cheap , that stuff don't last,ends up more$.
Went to snowmobile shop, seat covering is stronger and less expensive.
Wanted seat pan not of metal or plywood...cut apart a pick up truck bed liner ,
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cut it with a jigsaw, could mold it with heat ,
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drilled it out to take stock mounts. that seemed less expensive. and worked out fine for me.
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Otherwise, I save money by doing everything here.