I have a 49 jeepster & the springs are shot. New springs are costly. Can plywood bottoms with 4"foam & padding provide a suitable alternative?
Sure why not, use a treated plywood to avoid rot, and wrap it with vinyl, using a staple gun to affix the vinyl. Or have someone make a vinyl seat cover to go over the foam.
seat springs if they are the single wire that goes back and forth are available at many upholstery supplys... it does take specialized tools to bend them for length... but they are available... have a look at them at auveco.com you will have to download their catalog usually ..auveco is the company that has the automotive trim stuff in little green boxes..
I've see people tie the springs together with good heavy wire or really thick jip ties to give them some bounce back i'm cheap though i guess lol .
yes , plywood bottoms with 4 inch foam and padding is surely a proper alternative.You should definitely go for it....................car upholstery