Here is a link to the math in creating one.
Torispherical Dome equations
What equipment do you have available? How accurate does it need to be?
You could vacuum form the dome on a piece of hard wood, then trim the .04 thick material to suit.
How about metal spinning out of soft copper or aluminum sheet? For that matter, you may be able to do this by hand working the copper with a wood mallet on a form tool too.