I mounted the power supply for the red belly strobe next to the main one, using basically the same type of bracketry as I used to mount the ELT:
Astute readers will recall that this box is designed for ultralights, so as such it has less power output than a real-airplane strobe power supply. In this case that's exactly what I want, since I don't want my belly strobe to be quite as blinding as the real three-point anticollision strobe lights. I actually hooked up the 70821 strobe head and gave it a try before I installed this thing permanently. The flash rate is lower than the main strobe power supply, and the pulses seem less intense. Excellent.