FYI: I would not suggest mounting ballasts inside of the headlight enclosure. I did this and one of my ballasts died after only 45 minutes of night driving. I pulled over, popped the headlight out, and removed the back cover to check the ballast. It was so hot that it softened the insulation on the wires.
Mounting them to the back of the cover like the OEM litronics seems like the best way to go. Keeps it all as one single unit and allows air to circulate around the ballast.
EDIT: ballast is ok. It just went into thermal protection. I have since mounted the ballasts to the rear covers like OEM.