Vince, I went out and took a look at the box. There's some space at the top, above the rack, and there's more space at the bottom. It might be easiest to set the modem on the inside bottom of the box, below where the power supplies are, or attach it to the metal plate just below the power supplies. You've got about 3" H x 9" W x 5" D there at the plate.
You're right about the power supplies: we didn't use our regular PS5 or PS24 due to space issues.
Here's a photo of the inside so you can see a bit more. If you have more questions, give our Pre-sales Engineering folks a call: Toll-free in the U.S. & Canada: 800-321-6786, or 1+951-695-3000