to insert

I'm moving ahead with the plan to have the focus-mask sit just inside the tube of the Celestron 8-inch SCT.

I had wondered if it would make a difference where the mask was, in front of the objective. But I'm seeing that some designs (including those by Spike-a) have a donut hole in the middle so the mask can fit around the secondary mirror. That would draw the mask closer to the corrector plate.

On a refractor, I suppose, you would not want to take this approach since you could directly easily accidentally contact the glass!

I measured the inside of the OTA. The diameter is 226.5 millimetres.