[Yosemite Sam]

Years ago we used to "weld" rubber bumpers in the body shop, I guess they held up ok, I don't know of any that ever came back.

I guess it would depend on the location and severity of the split as to how much stress the repair would need to take and getting the right material or "rod" to use as fill. Depending on where the split is, spray flex-seal on the inside after your repair.

I don't know if you could ever get the repair to be water tight but I would think that it will keep sand inside unless it were a bad split or if it were to split open again.

Worst case, if it did split open again you could go back to your original plan and fill it with concrete.

Good luck.