Small porta potti - do you know of one?

We're looking for a small porta potti, less than 30cm high (i.e. lower than the smallest Thetford or Fiamma).  We are buying an Autosleeper Trident, which seems suitable in every way, except that there is nowhere for the porta potti to go inside the vehicle.  We really don't want to have to haul it in from the boot every time we need to use it!  Any suggestions would be very welcome.