Are there any pitfalls in this volume of mortar with chipboard floor and the fiberglass on the remaining surfaces? Any views/recommendations/tips e.g. lay polythene membrane over the chipboard, an off the wall suggestion "put the mortar in aluminium take away trays - crush down and easily placed"
Even sliding into place on removable battens is going to be a problem with 3 walls in situ and a mortar bed going from say 5mm at edges to around 50mm over the rest. If the top surface of the shower tray is 20mm thick would it be reasonable to assume that voids left by 3*2's pulled out after sliding the tray in on them would be insignificant (or in fact I'll need them to allow levelling of the tray) ?