I took these pics from Mattzilla's DIY on installing an HID kit.
Driver's Side
Passenger Side
If you look on the bottom center of each picture, you will see a black box zip tied to a metal frame. Those boxes are actually relays, I'm sure for this particular application which looks kinda like the Xenon Depot HIDs does, they come with several relay boxes. But pretty much thats where I mounted my ballasts.
If you have slim ballasts then I would suggest following what he did in the mounting locations but the normal size ones are awfully large making this frame ledge a perfect place to mount them IMO.