The box is big enough to hold a 14" disk. You can make it whatever size you want.
The top compartment has a lid that the LED strip is stuck on. The adhesive will not stick well on the mdf I used, so I put a track of superglue, the cheap stuff you buy at the cheap stores. Use a glove to spread the track of the stuff. The 5 metre strip of red LED's is cut to lengths
https://www.ebay.com.au/itm/183879660663 to fit the lid.
Each strip has marked where to cut it and you need to carefully cut and scrape away the silicon cover to expose the contacts. Stick down the strip and solder copper wire from + to + and - to -. Connect you power supply to the daisy chained + and -. the LED's point downwards.
The bottom of the top compartment is plain glass 3 or 4 mm with a sheet of plain white paper over it to act as diffuser.
The testing piece on the bottom and an optical flat on the top.
https://www.ebay.com.au/itm/323769219170.
The semitransparent 45deg glass is supposed to be so one can see the reflection of the test piece. The sheet of glass is to look at the test glass from the front. It did not work for me, it was too dim and did not show the fringes. I increased the number of leds and doubled it, but it did not help. When I get the time/inclination, I will remove the glass, no point having it.
Sometimes sodium light is recommended, but it is harder to get and more expensive and for my purpose the LED's work fine.
I hope that is enough for you, if not I go down to the shed tomorrow and take some photos.