If you look through the links in my sig you'll see that I did use a board. My amplifier has the heatsinks mounted to the sides so that inverting the amp won't effect it's ability to dissipate heat.
I don't quite remember what I used to mount the wood on the rear deck but it's probably along the lines of nylon spacers along with a bolt and lock washer+nut?
That's a very nice BA amplifier! Wish I could hear your installation. You picked a very nice sub too. There's nothing wrong with overboard if it gives you piece of mind. It also helps to line everything to prevent panels from rubbing. Otherwise, 25% coverage moves 90% of the resonance to a lower frequency.