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Glen,
I use many front surface mirrors in my DIY spectroscopes...
Obtaining the shape (Square/rectangular/elliptical) the required surface accuracy, and preferred coatings is an expensive challenge.
For my smaller mirrors I used Knight Optics in the UK, but I think you're on the right track with your thoughts of using secondary mirrors.
Bang per buck they give a good quality alternative and are readily available.
No optical issues other than the sizing of the mirrors (for required FOV) and the mounting plates (to give X-Y-Z alignment).
Sounds like an interesting project.
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