As you may already know, that stress cracking comes about because of misalignment of the bumper bracket mounts on the rear spreader bar under that tank cover. Rather than mess with trying to fortify the cover I'd suggest you work at bending the tabs on the spreader bar so they are nearly right up against the back side of where you patched, I say nearly because I like to put some antisqeak material in there too. If the spacing can't be tightened up without a bunch of modification to the under brackets then some shimming might be necessary. It can be fun to get the bumper mounted straight on those sometimes.