I built an enclosed stand up camper trailer using Poor Mans Fiberglass (PMF). I was in a hurry and had looked at bonded plywood/ foam/aluminum (SIP) panels as a base because I was concerned about strength. I did what everybody said not to do, couldn't be done and wouldn't last. Instead of high dollar and heavy SIP panels or even 2" foam sheets at $40 a 4' x 8' sheet I used $20 interior doors. At 30 pounds a door not as light as foam but super rigid and lighter than SIP panels. Put 1/4 " plywood on them and fill them with spray foam and my 200 lbs can jump on them. Use the sock method of PMF and you would have a hard time getting them to break with a hammer, but most importantly it completely waterproofs them. I still haven't tried aluminum screen wire between the cloth but it is bound to be like rebar in concrete, even if you could crack in it wouldn't come apart. I have no doubt you can build a better top than you can buy new. Paul Elkins built a tent type shelter that slots and is duct taped together out of the cheap one sided foil backed foam that he stands on top of for burning man photos. I have pulled my trailer for almost 10,000 miles at highway speeds with no problems. I backed into a 4" limb which when it broke off and the jagged end dented the foam but didn't pierce the cloth. I dropped a corner of a 1' square steel tubing scaffold covered with 1/8" expanded metal from about 6' above (over 100 pounds) it pierced the cloth, outside of the masonite door coating but not the inner and took about $2 worth of materials to fix and not including the drying times 15 minutes to fix. I would guess if you wanted to build something insulated to keep out the elements it would cost maybe $500 and that includes what ever tools you might need. If you wanted something to mount solar panels on and be able to walk on maybe $650. If painted white or some other forgiving color it will be mistaken for fiberglass by most people. I think fitting it to the roof would be the most time consuming part. Junkyard could probably supply a used fiberglass topper cheaper than you could build one but you could build a much sturdier one for the same money and a lot of sweat. PM me if you can't find answers in tntt forums. Unlike fiberglass this only requires rubber gloves and a paint brush. This is a modern spin on 1900's tech.
