We use MK1's Mac Mini shelf (as seen at http://photos.imgix.com/building-a-graphics-card-for-the-int...), which is pictured at that page. I just wasn't in love with their Mac Pro shelf design, so we went a different direction.

Keep in mind also that 8 Pros in 7U = 48 in a 44U space. So it's a pretty similar density, but I don't think it is as ideal in terms of airflow. Instead, it's more ideal for working on the systems individually (such as in a colocation environment), but that isn't a particular concern of ours.