On current version (1.2.4), the crafting slab is not craftable due to sharing a recipe with the standard crafting table to convert the full size station back to the stock block. Best solution would be to either remove the ability to craft back or make it require the full size station plus a piece of wood or similar.

In 2.0.7 is craftable, but the Crafting Table Slab needs 3 Crafting Table to create only 2 Crafting Table Slab. I sugest change it. May be a recursive craft: Crafting table => Crafting Station => 2x Crafting Station Slab => 2x Crafting Table Slab => Crafting Table