After trying different plastics in the gap, with an LED shining through the backside; I decided that the gap between the center and outer section wasn't big enough to let in enough light. I unsoldered everything then took some off the OD of the center section. Did this by tightening the button "bolt/nut" through the center section, chucked the threaded end of the bolt/button in the drill and held a file against the OD while spinning the drill..
After resoldering the center to the outer section, I put the bolt/button back through the center, re-tightened, then put the bolt threaded end back in the drill and filed off a stepped sholder from the backside of the outer section. That gave me a little more thread sticking out the back of my panel. After cutting down the backside of the first button, I started liberating parts from the other faucet valve/assembly to make another one
I didn't have the same pipe/compression fitting for the center section, but I had one with the same OD/ID that was pipe threads on both ends. I'll have to make a grove/shoulder at the base of the small pipe thread side, so the trusses have something to sit on/into, before soldering.
Here's a new picture of the first button, showing the wider gap/cleaner soldering. It's sitting next to the old faucet parts that I used to make it and another picture of the other faucet valve parts, along with the nut/bolt and pipe fitting that will make up the button and center section.