The light is D-shaped with the batteries on the bottom. I took the handle off completely. Design looks like Fatty hippy's drawing now.
Battery modules looked really cool! Everything looks "plug 'n pRay. Ok for a newbie!

Maybe a long zip tie can go at the base of the ballast to clamp it to the bike stem. The battery can sit in the seat bag.

But waterproofing seems to be the problem. The ballest looks watertight. But the clamp opening, battery, and switch need to be waterproofed...

Any suggestions?