Dear Stephanie,
As for the Besseler 23CII, I can not speak, but I was told that Besseler and Omega were build quit the same.
So Here are some examples of how I modified and build what I needed for my Omega D3.
Here in Belgium, spare parts for Omega's are hard to find, as a matter of fact, I was lucky to find one at all.
As you can see, the modifications on the lens carriers were done in black acrylic glass of about 2,5 mm thick (= 1/10 inch?), black plastic tube, black tape and 4 mm nuts and bolds. The Acrylic glass is 100% opaque and easy to be worked at and rather stiff enough to be used as such. The nuts and bolds can be used to trim the parallelism of the lens flange and the carrier.
The picture with the Schneider 105 mm shows an original lens carrier.
For the negative carrier, I had to make for 6,5 x 9 cm sheet film, I used some aluminium of about 3 mm thick. I had to cut two sheets the same outer size, but one with an opening the same as the size of the image and the second as large as the film sheet, and stuck them together with some 4 mm bolds. The over all form is copied of an original Omega negative carrier. The film is simply resting in the carrier, then I lay a 6,5 x 9 cm sheet of anti Newton glass over it to prevent it of curling upwards due to the lamp heat, this is why i did not use the acrylic glass for this item.
I hope you will find some inspiration in these examples.
Good luck with your Besseler,
Philippe