I bought this seven segment display from The Pi Hut. There is a reason, as well as just being cheap.
It's red, which is good for B&W paper (there will be an option for zero display output however). It also has four decimal point lights, rather than a clock-style colon. I'm going to use this as an indicator for which timer is currently active (there will be four), so I can preset enlarger, dev, stop, and fix timers.
It's common cathode, which means it doesn't work with the advertised TPIC6B595 (I think I read it wrong). Thankfully though, Arduinos have just enough pins to allow me to operate all the periphery.
Unfortunately, the fourth digit was being weird, so I need to look at that. Maybe I wired something wrong.
Meanwhile, I got the relay clicking (small wins) and managed to read the rotary encoder. The red backlit safelight button I plan to use has got a nice dim red, so that is great.
I got some initial plans drawn up for the body in Onshape (my CAD program of choice) and I have some black filament on the 3d printer.
My only challenge now is how I am going to output wires for the LED strips - I think a stereo audio jack would work. I also need to think about how removable it needs to be, with the 240 volt connection.