For the mounting board (board between control board and power board), I just use a piece of FR4 with no copper on either side. See pages 57 through 63 for a step by step of how to mount the control board to it, adn then how to mount it to the power board.
http://www.paulandsabrinasevstuff.co...il,%202011.pdf
I use 8" x 6" unclad FR4. Ebay sells it, or you can get something like it at radio shack. 8"x6" is a pretty common size. Just put the control board on it and mark the mounting holes, adn then drill them out. Then drill the 10 gate holes and 2 ground wire holes. Precision isn't important at all with it.
Make sure you don't connect the aluminum heat spreader directly to the power board. It needs to be electrically isolated from the power board.
The steel bar is just for re-inforcing the bolts, so you can clamp it down nice and tight. You can use copper or aluminum or anything you want.
Good luck!