Shuttlepilot, that's a slick solution. I thought of doing that on every one of the interiors I've done and in the end skipped it in favor of more cabinet/drawer/closet space and just put the porta potty in the back and strapped it in.
My family (or should I say my wife and daughter) love having a PP on board
One thing that's not obvious when planning where to put it is how much shoulder room you really need while using the thing. If you're an average size adult you need a little wiggle room above it which is why the rear center has always been where we set ours at night, or between the front seats which we found to be the best place. It's amazing to me how little smell, really no smell, modern PP's have so we've never minded putting it there, but there's always a little paranoia by yours truly about a spill or splash right there around the front seats. OK, sorry, it's a crappy subject.