OK some reinforcement. First step was to delete that pesky hole, with a little fiddly piece of 3mm plate shaped to fit into the hole and given a beveled edge:



Weld that in, on both sides:



And then sand it flat (with the "dynafile" type thing):



Now we can get a decent fillet in all the way along the bottom:




And then add a gusset:







That's not going anywhere.

Finally I cut the post mounts off carefully with a Dremel and sanded them flat: