http://www.thingiverse.com/thing:1294699
This a modular holder for standard optical mirrors (25.4x25.4x1.3 mm) used in spectroscopy.
Mirror holder volume: 41x41x33 mm.
Solid sample disc adapter must be resized to a factor of 0.9 in order to fit the slit properly.
Solid sample volume: 9x27.9x48.6 mm.
Additional plate support for other mirror sizes and geometries (elliptic, rectangular and circular mirrors).
Source code is parametric, so you can easily resize, modify, distort every detail of this housing to best suit your needs (ie. to adapt it to a different mirror size and geometry).