ReefEnabler, the bulbs have a "cold spot" on the end with the label. That's where the air is directed through the holes in the fixture, and why there only need to be holes on one end of the fixture. If you're doing a retro be careful not to blow too much air onto the bulbs. I talked with Grim quite a bit before doing my own fixture, and he pointed out that it is much more harmful to the bulbs to over cool them than it is to under cool them. In the ATI fixtures the air is pulled in through the top, runs by the ballast, and then down through the holes to hit the cold spot. So, the air has been heated before it ever gets to the bulbs. You only want this "cold spot" to be like 113 degrees F, not anywhere near 75 or 80 of room temp air blowing directly down on the bulbs. In fact, if you're talking about a true retro, not an enclosed fixture, you'll probably have plenty of passive air movement inside your hood to keep the bulbs cool enough. If however, you're talking about mimicking an ATI fixture, then just keep that layout in mind and direct the air across the ballasts before it gets to the bulbs.
My fixture, is 10 bulb like I said before, Icecap ballasts (the only good reason I made my own), 4 fans directed across the ballasts then down through a slot to pass over the bulbs at the cold spot (the fans are temp controlled too, so they don't turn on prematurely), reefgeek reflectors (smaller footprint than icecap and slightly better), icecap endcaps, SS body like the powermodules.
Really, the powermodules are a very good deal, unless you want to overdrive the bulbs, especially reefgeek has great prices on them. Also, I heard ATI has a new fixture coming out that will overdrive the bulbs and will be dimmable too(I think?). If I'd been able to wait, I would have gone that route since the overdriving aspect was the only thing I wasn't going to get from ATI, because building my own was a huge PITA, and not cheap.
