So we definitely want to make sure nothing fires while putting things together. To that end, the relay boards will be powered separately from the RPi, and the actual firing power will be on yet another circuit. My junk stash yielded up this box, which was previously used for an arduino-powered quiz button system:

The existing nintendo power button will switch 12v to the relay board, and a salvaged printer power switch (the green blob in top/left) will switch firing power. Firing power will likely be 12v but it could turn out to be 48v if voltage drop is too high. I'll be adding binding posts for each circuit and screwing the box to the main sheet of plywood.