I redesigned the cuvette holder finally, this is just the prototype but she works great. It looks a little ruff I know, but this is just in the "proof of concept" mode. The cuvette holder is made from a high density, non-compressible black foam material.

The cuvette fits nicely in a collum cut at 12mm square @ 37mm in length down the holder's middle, so 3 sides are completely enveloped so light has no were else to go but back inside the cuvette and back out twoard the detector's slit.

Even though the cuvette's themselves are only 10cm path length, I cut the extra 2mm so it slides in and out easily.