Start with a piece of copper (I used 24 gauge) measuring 6 1/2" x 6 1/2". In the center, mark a 1 1/2" circle and saw it out.
This is the sheet that will sit over the hotplate (once the hole is cut).
Next, measure and cut a piece of 2 1/2" x 2 1/2" copper. Again, mark in the center a 1 1/2" circle. Anneal this sheet.
Using a dap and punch, make a concave dome in this sheet.
Place this smaller sheet inside the hole of the larger sheet. Now you have a curved area to keum-boo on that should be pretty stable.