RE: Does anyone know of a source for a precut firewall?
Cutting boards are VERY easy to cut with a simple hand saw. As for the shoulder, you can use the hand saw to cut a small grove around the edge where you want the shoulder, and then use the hand saw to cut to that grove. You can probably have it completely cut in 15 minutes or less. After rough cutting with the saw, you can use a utility knife to clean up the edges or make fine adjustments.
Brad