Someone posted the name of a company in Buffalo that makes inexpensive monochromators. I did not save the information as I already have one! Sorry. However, IIRC, they ran under $50.

In my unit, it is a fully straight line path from the light to the "output". The monochromator has an opening and is box like with an opening on exactly the other side for the exit beam. There are probably many twists or turns inside the "black box" but I am loathe to open it. It works you see. So, what ain't broke, don't try to fix.

There are 2 photos of the unit in the book and a sequence of the unit in operation in the DVDs.