It’s done. Total cost: $75 ($50 for new tires, tubes, $25 for velcro). And a lot of time. The fabric is rip-stop nylon (donated by my mother-in-law). I used sticky-back velcro to wrap the fabric around the frame. All the fabric will come off for folding/storage. I also attached an old rug on the floor of the trailer to stop stuff from sliding. My neighbor Kathleen did the sewing for me, I cooked some black beans in my pressure cooker for her in exchange. It takes a village! Very satisfying project.