Mount them outside of the canopy. Most ballasts have a max "permissible" length of wire that you can use to connect your lights to it. If that length is long enough, I would mount the ballast inside my cabinet, near the top in a dry location (run the wires inside flexible conduit). <Perhaps on a movable board/case for easy removal during maintenance.>
External (to the fixture) ballasts means they are not exposed to moisture or heat inside the canopy, prolonging life. Also, not having the ballasts in there makes moving the canopy easier.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.