Mojavegold,

I use Homosote...its 1/2 inch thick, white on one side and grey on the other. The commercial application is for sound barrier and also for ceilings. It comes in 4X8 sheets.

Also, if you could get a peice of old pool table slate to build off of it is the best. Put the Homosote on top of the slate. Definitely flat...really flat.

Level is level....but flat is more important.

Tom