To be fair my friend had some aluminum bar stock that I cut off, cut diagonally, cut a corner off of, and then drilled a hole through it.... Very little measuring.
width 3 1/4, side 1 1/4, length 3 3/4
You really don't need anything remotely this strong. Plywood would work fine. It's not structural and only the top is visible when installed. The only purpose is to keep the retractor straight up/down.