[ChrisH]

I couldn't find any either and I checked the usual suspects.

I guess we just made our octogon pieces. They are not high precision. The way we mount the FP gearboxes has lots of slop. The ones we made were 1" or 1 1/2" thick. You could machine them out of square stock if you have a mill.

If not you can use a band saw. Lay out an octogon of the proper size on the aluminum plate. Cut the shape out wit a band saw, taking care to make clean cuts. It is OK to be slightly oversize. Then file down to the layout marks. If you really have to, you could probably use a hacksaw, but it is hard to get a good cut that way.