I have all my equipment in Middle Atlantic racks. The 990 was an extremely tight fit, due mostly to its depth. The BRK-20 rack is deep (18") but as you all know, so is the 990. I had to cut away some of the plastic channel that I use for cable management, but after that It slid in fine.

I have all my components on Middle Atlantic U2 racks. The shelf is wide enough to accomodate the 990 and the capacity is 50 lbs. Although the shelf is only 14 3/4" deep, the 990 sits on it ok. The feet are inset enough for it to sit on the lips of the shelf. I've seen that Middle Atlantic offers custom shelves but don't know how they look or how much they cost. The U2s look and perform fine for me and at less than $40 each, do so at a good price.

Here's a link to pics of my gear with the newly acquired 990 and 7100 sitting proudly at the bottom:
http://pg.photos.yahoo.com/ph/fletch71/album?.dir=1861&.src=ph&store=&prodid=&.done=http%3a//pg.photos.yahoo.com/ph/fletch71/my_photos