Why not make custom brackets out of either mild galvenized steel?
Run them around the rod case, and also around the rear subframe tube to be sure the rod case does not fall loose in the event of a tab breaking free.
On the end near the rod case cap, a simple solution exists too. It just takes two bolts, and the same strap material. Run a small loop around the frame tube, and a large one around the case near the cap. For extra material, you could just double up the strap between the rod case and the frame tube. (two layers thick, held together with a few bolts.)
I don't have the stock evap system canister clamps. I guess I need to just try this, or buy two of the larger clamps... LOL
Time to go rod case shopping. (Need one in white, so it should be easy I hope.)