Step 4: Stiff board
I sandwiched the stiff board between two boards (about 3 1/2 inches long, and slightly less than 3/8 inch thick) and bolted the whole thing together. It's very similar to how a knife handle is held on. The width of these boards is slightly larger than the width of the stiff board or gear. This gives the gear a little bit of breathing room to spin freely.
You can then hold the whole thing together, and drill through the two sides and the middle piece and hold it together with the same wooden peg method we've been using. I kept my pegs toward the top, so the hole ran through only one of the sandwich pieces, rather than running through the stiff board.
I should mention that before making everything permanent, you should check the length of your stiff board. It should be short enough that it is not rubbing against the gear at its thinnest point. But it should be long enough that it slaps the next gear tooth after slipping off the previous one. Experiment a bit to find the optimum length for maximum slappage.