Hey Joe,
I use a Yakima Pro 16 cu ft on mine. Here's a couple of shots from my gallery to give you an idea of what it looks like...
That last shot is from moving out of our ski house at the end of the season a couple of years back - there are 2 snowboards, 3 sets of ski poles, at least four pairs of skis, and a few boots in the cargo box. It's great for camping too - all the light, bulky items (tent, tarps, sleeping bags, chairs, etc) go in there.
My Yakima box attaches with built-in clamps on the bottom of the box that fit our cross-bars perfectly and make mounting & dismounting a snap - I can do it without any helpers in a couple of minutes with no tools.
I think Yakima discontinued that model line and replaced it with the Skybox Pro line (the Pro line comes in silver to match my Astral VX).