I was able to get a single-kid trailer nearly free because the fabric was all moldy. I unfortunately don't have pics of what it looked like to start, but it was one of the ones where the frame was collapsible. I stripped the fabric off and rearranged the tubing to get a low rail around it (see pics). Note that the foot-well bar is reversed so it's a low front rail rather than sticking below. I think there was one pair of tubes that I had to drill new holes in, but otherwise I just used the existing tubes and hardware in a different configuration. The main mod was putting crossbars in to make the base into a platform. I used PVC T-junctions and some old aluminum tent poles as tubing, then topped it with a double layer of coroplast for a light weight platform. I also have a plywood deck I can swap in for heavy stuff, but this works well to carry a couple of rectangular rubber tubs (bungee cord across top to hold them in place). Good enough for a large grocery run.
Here are some not-very-good pics:
Top frame (attachment arm is folded underneath - white box under front is just to support it for photo, not part of trailer)
Underside with view of added cross rails. Wheels have been removed for better view. Black is tape to keep PVC junctions from sliding.
