I'll be putting a dual PII in it. This might require more height than 2U will provide, but 2.5U or 3U would be fine. I just want to hide it as much as possible.

It's a Tyan Tiger 2 (S1692D) mobo. The manual claims it's ATX size, and, more specifically, 9.5" x 12". It's perfectly rectangular. No cut out corners.