Thanks, Nate.
I fiddled some more to make the rig less elastic and got this alignment after a few minutes:
This is my current setup:
The camera sits on a Novoflex macro rail. The copy stand is a Fujimoto “Lucky Enlarger 90M-D” with its head removed. As we can see, I had to put in a shim in order to align the object and image planes. The shim is simply a piece of paper that I folded as many times as was needed to get the alignment. Later, I put the shim under the arca plate, which makes the setup look nicer (nerd alert ;-))
The macro rail allows me to copy 4.5x6 negatives as well as smaller formats without having to move the whole assembly. The mask on the light panel is a temporary model made from cardboard. For easier operations, I plan to make a similar thing from plywood so that I’ll get some distance between the backlight and the negative, which makes the setup a little bit less sensitive to dust.