If I really didn't want to spend the $$$. If there was some good flat metal on the underside, under the weld, I might think about a carefully crafted gasketed backing plate. I would have to fold it to get it through the hole, so it might be multi-layer. Alternativly, I might add a larger access plate nearby; always a handy thing, if there isn't one. Best to place it on the center line, where there is less splash impact.