When I built this, I slid the two arms together using my router base as a spacer, then I screwed 1/4" plywood to the bottoms of both arms. I cut through the plywood with a 1/2" bit.
If you sandwich a board between the two plywood "shelves" & tighten the wingnuts, you can cut an exact fit dado using the same bit, guiding the base against both arms.
