My wife tried out the passenger seat that I made about a week ago. Rode about 130 miles on it today and no complaints, about the seat anyway. Felt like riding in an oven though.
I think the total price in materials was about $80. Marine grade vinyl, 2" high density foam, piece of 1/8 plate steel, fabric sealant, elastic and upholstery thread.
The shape needs some work and I might change the vinyl later but overall I'm satisfied for now.
Photos are in my album. Could not figure out how to put one here.