You might be able to figure out a way to make a "T-nut" of sorts instead of the round rivnut. A bar of say 1/4" thick steel that will slide into the channel beyond the rivnut area, going both ways. Drill and tap the appropriate hole for the bolts. Then slide it in so that the hole is centered over the rivnut opening. The bar will distribute the forces over a wider area of the plastic. Even if it is only an inch in each direction it would be more support than that small rounded area.
Perhaps you could make it one long bar and slide it in from the end. Drill and tap the holes first to make sure they line up with the bolts. Then cut a slot at the end of the channel and slide it in. Once aligned properly figure out a way to secure it enough that it doesn't move while you put the roof back on.
Personally it looks like the manufacturer kinda cheeped out on the structure of the top, making it too light to do the job. If it will blow off on a trailer, it might do the same if left outside in a storm. Many products these days are just not as robust as they should be. I think you can fix this for way less than a replacement will cost, if you have the time and the skills. Buying a replacement is always the backup plan.
As backup plans go, I would probably always toss a strap over the roof when moving on a trailer and maybe even if it is parked outside with a storm coming.
Good luck with your repairs and please post pictures when complete.