A few months before we left, when we were still deciding what system to use to carry our camping gear, we found a discarded Thule rooftop box laying on the side of the road. There were big cracks on both sides but otherwise it was in decent shape. The box was locked when we found it so we paid $5 to get new keys made at a locksmith. We were a little afraid of what we might find inside a discarded locked box that we found on the side of the road. To our relief, it turned out to be empty and shiny and clean inside.
After consulting google, we decided to attempt the repair using ABS cement and fiberglass strips, the kind used for drywall repair. We found the ABS cement in the plumbing section of Lowe's.
We lightly sanded down the area of the crack extending 6"-8" all around. We applied a layer of ABS cement to the surface of the box, laid down a layer of fiberglass, and brushed more cement on until it was saturated. We repeated with a second layer using a slightly larger fiberglass patch. Depending on the cement you get, the stuff may set up quite fast. Be sure to have the fiberglass patches cut and ready to go in advance.
Once the repaired area dried, we painted the top of the box with Plasti-Dip, an aerosol plastic coating. I think regular spray paint would have worked just as well, but someone on the internet suggested Plasti-Dip and we went with it. We took the opportunity to paint the top of the box white, to reduce heat attraction and keep the temperature inside the box down. While we were at it, we made stencils and decorated and personalized the box a little.
Certainly the repaired portion of the box didn't look as good as new, but it held up to a year of constant use in extreme temperatures and humidity. And coming across a free box solved the dilemma of what cargo carrying system we would use. If you've learned anything from reading this site, you'll know that free is our favorite type of thing.