I found myself in a similar conundrum today. Thankfully I found this thread. I was able to rough-cut half inch nylon (or whatever the plastic is) and then using a belt sander and patience, get a rough octagon that fits snuggly in the FP gearbox.
Then there's getting the hole in the middle. That's the real pain, if you do it rough like I did. I tried using an old FP gearbox (we have a dozen, I think) as a guide for the hole. Didn't work too well, wobbles a lot and I had an off center hole. I also had the entire gearbox swinging around when it got jammed on the 15/32" drill bit that was in the drill press.
I'm glad that I, a mentor, didn't delegate that task.
I'm going to redo both octagons it using a turntable that mounts on the mill's table. (drilling the center hole first)