i looked into building them myself. but you pretty much have to do it like a real business . in order to buy supplies at wholesale and get good prices on the parts and you also have to buy it in a larger amount than what you need.
and getting the frame built is expensive to. every shop wants exact measure ments and a one time set up fee. i think 25 frames was gonna cost me $925.00 + shipping and if they didnt fit it was my own fault. then you got to buy canvas , zippers, windows ( cut to fit ) , straps, thread, snaps, etc.
then make patterns for the top so you can cut it out. then sew it all up .
and if it dont fit , take it apart and make adjustments until its right. then when its right you take it apart and make new pattern from that one. throw the old pattern and top in the trash and then start building them . its expensive and very time consuming. and the profit wouldnt be close enough to make you want to go through it all. then you got to hope you dont get taken to court for copyright laws from another co. that doesnt even make them anymore.
and back to the business part of it. the only way to purchase supplies at the lowest price you need a tax id number. which means uncle sam wants his share out of it. and a business id #.