8 3/4 Save or trash???

If you can heat up a piece of metal (1/6" plate), you can hammer it against the outside of the dome to get the shape then finish it on the inside (which should be close enough). It doesn't have to fit perfect but all the edges should be making contact to get a good weld. Also, be sure to remove every bit of corrosion so that you get a good solid weld!!
There's nothing that can't be done but some things take a little longer (think Pyramids).
treblig