Ok, more clarification needed I think.
The walls are square, however there is a gap. I pushed the shower tray tight to one end where it sits square along its face. I’ve measured the gap correctly this time and I have between an 8mm to 9mm gap.
In theory if I reposition the tray this leaves a 4-5mm gap either side.
Tiles aren’t being used as previously mentioned as I will be installing shower panelling. The panels are 10mm each plus the dot and dab adhesive will give me 12mm for each panel.
This means each board will be overlapping the tray by at least 7mm.
I think I will scrap the riser tray and lay it on a bed of cement over a ply raised plinth.
Now will this still be a compromise and should the above be OK?
Leak prevention is number one on the list