Bungee is a good workaround -- annoying that you need workarounds, but still.

I came here to say "drop bolt" too; this is what we did on my trailer and it's done fine over the bumpy Cooper roads for 20 years. Granted, that's still lower mileage than you're seeing; mine's not road-legal. But the Coopers are pretty rough on trailers too. (They don't really care; it's not their property.)

I'm not sure how deep they are, but it's significant. I've got them at the bottom and top.

Good luck finding the right permanent fix for you!