I think you might take a look at what Ciadella Interiors has.
They have Cameo seat covers but also sell just original style cloth material and vinyle. They have seat foam if you want to go that route. I think the original was some sort of cotton batting and straps.
Ciadella Interiors I bought some covers and door center liners and seat back covers for my 57 3100. Looks pretty good to me but have not tried to install it yet.
