My approach would be to reshape the piece with files after welding.
If you’re still using grandpas files go to an Ace hardware store and
get some new Nicholson files. Its what I use to reshape coil
laminates and a new file goes through the steel like butter. Then
drill the hole from the bottom when you have the proper shape. I think
that is what Dave was suggesting. Its going to be a challenge to
find a way to hold the cover so you can work the cover if you do
decide to file it.
Tubs.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.