Jeff:
It's all been done. I designed one like that some time ago. It's designed to fit along the wall in the back of the van, but will fit elsewhere. It's ported @ 3", but can be bushed down to 2.5", or 2". (An option is (2) 2" ports.)
The input side has a 2.5, that comes bushed @ 2" but is removable. (An option is (2) 2" ports.
The design pulls from the bottom of the container, yet the port(s) to the system connects midway up the height of that side. (That keeps the hose off of the floor, and allows it to span over a wheel well, in the against the wall installation.)
They're more expensive than what I've seen others do ($795), but they are more versatile and provide better airflow.