I think I need to write you back on email. dang it! I read it, but then got busy. haha. That looks like a awesome layout. You save about an inch by raising the igbts like that. It's basically optimal I think. They do sell copper sheeting in any thickness you want. I'd try a place like this:
Nimrod Hall Copper Foil Company
They have thicker stuff too. 10 mil is around 8 or 9 ounce copper. Then you can just add some flexible isolating tape to one of the plates. Very cool!