To avoid the use of a diagonal I turned up an adaptor from a piece of aluminium that takes the PST eyepiece unit and allows a camera to reach focus without a diagonal, so there’s nothing else in the light path. I shall use this for my imaging, though I’ll revert to the diagonal for visual. I’ve been fortunate enough to obtain a second eyepiece unit, so I don’t have to keep swapping them over. Here’s the adaptor:
And here’s how it looks in place on my modded PST:
I continued the thread for the eyepiece unit all the way down the inside to try to prevent reflections reaching the camera. Now I shall paint it black using heat-resistant barbecue paint.