I understand the frustration that has lead you to a portapotti. The old Brydon Boy's are a curse. I remember the satisfaction of taking a chainsaw to the old factory-installed head.
When I changed mine I did some research that made me think that a Simpson-Lawrence head was intended for the Contessa as the bowls are relatively low. These are no longer in production and were really a UK thing.
I now have a vintage Raritan PH II which works like a charm. In the Gallery section there are photos of several head installations.
The real challenge is that the Porta-Potti is intended to sit flat and the Contessa has too much curvature in that area. If you are seriously pursuing this I would relocate the head under the V-Berth. You won't be able to use it and the V-Berth at the same time but the Porta-Potty should fit.
As usual, measure measure measure.
Good luck!