To test this.. Do it with Covers or Panels..
There are two different slabs in the game.. vanilla ones that are made with 3 blocks.. And Redpower 2 ones that you have to make with the saw.
Block>saw=slab, slab>saw=Panels, panels>saw=cover.
8 Covers or panels or slabs in a crafting table, empty space in middle = Hollow slab,panel, or cover.
Same way you make a chest.