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A very doable project, Chris. I have had similar ideas a couple of times in the past when interesting lenses have come up for sale. The large IStar lens in the classifieds is an ideal starting point for a folded optics solution!!
Will the unit be for terrestrial or Astronomical observing? I know my body will not let me observe using straight through refractors any more.!
If it was me I would fix a 90 degree diagonal to the box to allow for positioning the eyepiece for comfortable use. The alternative would be to insert a third mirror in the box to achieve the same outcome. The eyepiece would be attached to the top of the box.
I think the best first surface mirrors reflect about 95% of incident light - that is, there is about 5% light loss. Your two mirror design will have light loss of just under 10%. This level of loss is not easily detected in a comparison with a straight through refractor of the same aperture. If the test refractor has a diagonal, then the difference is only 5%, which would not be detectable at all. With my suggestion of a third mirror, the light loss climbs to 14.3%. However if the comparison is again with a refractor with a diagonal, the relative light loss is again just under 10% and not easily detected.
Shall be interested to see this project unfold.
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