Brilliant effort David.
The system looks to be delivering superb image quality that would otherwise be extremely expensive.
I've had some discussions in the past with Peter Ceravolo (Canadian master-optician mate of mine ) about secondaries.
Yes, as Mark suggested you can run a curved secondary but instead of the energy going into the diffraction spikes, Ceravolo says it goes around the stars and uniformly and hence drops the contrast.
If you really want the whole shebang, an optical window is the best (albeit not cheap) way to go.