I think the easiest way is with a 6 or lowside and simple wardrobe.
Build a simple plynth (flat bit of floor) where the wardrobe is to cover the wheel arch and form a base big enough for the small porta-potti. This will then be a comfotable height to sit on and the potti will travel in the 6 when folded down.[You must be registered and logged in to see this link.]
Use the wardrobe as an enclosure, perhaps by moving it out slightly by lengthening the sides with extra lengths of plywood fixed with piano hinges. This will then fold so will lay on the floor.
Inside the wardrobe door, add a second piece of plywood, approximately the same size as the door. Fit this on one side with piano hinge so it will fold out wehen the door is open to form a Vee shaped space in front of the potti. Hopefully this should create enough room to use the Potti.
This means no holes in the side and not too much space taken up when potti not in use. A cassette toilet really is no different in use to a porta-potti. The only difference is, you can empty it from outside.
You can buy faced marine ply, probably 3,4 or 5mm thick which is very strong and has a finished wood fac which can be varnished etc.
Probably best not to have a basin as that complicates things.