I fitted the stud wall around the base of the toilet and shower for an exact fit.  I then made a box out of ply to fit the shower tray and cut a hole in the floor for a pipe and drainage.  The walls were then clad in PVC wood effect cladding and sealed with bathroom silicon.  The shower is powered by a separate 12v pump that sits in a bucket of water that can be warmed up on the stove.  The toilet is a cassette toilet that pulls out to be emptied when its full.  We decided to keep the weight down and have a shower curtain instead of a door, using some left over copper piping to make a shower rail.