SAS,
why not just braise the boiler base to the 'plate'?
If you need to remove the boiler base for some reason (I can't think of one actually; any descaling can be done from the top) just do it like you would if you were replacing the heat element. Wouldn't that work?

The boiler is connected to the plate the same as it was connected to the original base, i cut a 3 3/16" hole in the plate with a jig saw. The brass plate I used was 1/4" thick, the original base with the gasket is only 1/8' thick, so I had to machine 1/8" from the bottom of the plate around the hole I'd cut. I used a 5" grinding disk on my drill press.
Yes the boiler could be brazed to the plate but this way if I want to convert it back to it's original, except for the heating element (which was burned out) I have all the parts.

alex