The spectrometer is made to accept magnetic[1] click-on sample accessories such as the spectroscopic cell holder:

1. Print two magnet holders (and maybe check that your magnets fit), and the cell holder’s two parts.
2. Insert the magnets in their holders and mount one holder on the spectrograph, and the other on the cell holder:
   

   
3. Assemble the rest of the cell holder:
   

   
4. Click it onto the spectrograph:
   
   The four 12mm M3 screws you see makes for an identical coupling between the spectrograph and the cell holder, and the cell holder and the lamp-house.

[1] If you feel magnetic click-on stuff is too fancy you may be right. However, it was the only solution I could come up with for a system of interchangeable sample accessories that would likely survive in the greasy hands of the teenagers I teach every day.