For info I once had to replace a broken resin 48 x 30 tray without damaging the laminated wall finishes.
This was achieved by firstly breaking out the damaged tray - which had originally been set on a 1/2" layer of cement on a full wooden floor.
All cement was removed and the floor was drilled extensively with 1" damater holes (you'll find out later)
A couple of long wooden slides some 3/8" thick were laid onto the wooden substrate. These permitted the new tray to be slid into position.
Next problem was how to "jack up" the tray to meet the existing shower walls. To do this a couple of bicyle tubes were slipped under and gently inflated. These were more than adequate to lift the tray into position and permitted extraction of the two slide strips.
To fix the tray into the elevated position a controlled amount of expanding foam was injected. Too much foam and there's a danger of the tray base buckling - the exanding foam is capable of exerting an amazing amount of pressure - been there and done it
That's why the floor was drilled, the holes offered an escape route for excess foam.
The result was a fully supported replacement tray with no damage to walls
Obviously I needed access outwith the cubicle to get the drain fitting installed but that was a simple job of cutting an armhole access point in the adjacent floor.