I went ahead and ordered the PCB's.  Trying something totally new this time.  Rather than fully wire the whole thing up, I just decided to figure out the mechanics of it (it's a really simple circuit, lets be honest here) and then draw the diagram and order the PCB, so the first test will be with the PCB.  Excited to see if this plan works.

In the meantime, I still needed brackets to attach the sensors to the DI cartridges, and ideally ones that block ambient light.  So I did some quick measurements of my BRS dual DI setup, and made little rings that slip onto the cart with a friction fit, and then position the sensor against it.  Did a test print, fitted the sensor in, got the alignment just right on the first try, and it seems to hold well.

Printing two in black right now..  Now I'm exited for all the bits to arrive!