Buy the spectroscope from AstroMedia and insert the lens, the grating and the scale into this selfprinted case. Or if you have a damaged Astromedia spectroscope, maybe you can bring it back to life with a new housing.

The lens is glued into the lensholder. The grating is glued to the other side of the lensholder.
The scale is glued onto the inner side of the frame. Pay attention to the orientation (see images).
To disperse the light which shines through the scale you can glue a strip of thin white paper (85 x 19 mm) onto the outer side of the frame (do not cover the light slot).

If in doubt, consult the original building instructions from astromedia.

For a first trial, you can use adhesive tape to hold the parts together. When everything works, use glue to make it durable.

11/10/20: The engraved writing (lid.stl) was too deep. Now it's 0.5mm instead of 1.0mm.