Recently, I replaced a shower/tub combination with a shower. I purchased a fiberglass shower tray.  It did not have very much support and when walking on it, would creek and pop a lot.  It was recommended to set the base in mortar to give it more support.  I did not want to add the extra weight.

After a bit of thinking, I lined the bottom of the tray and the floor with plastic and used triple expending foam between them.  After spraying out about a third of the gap space with the foam, I installed the tray over it and secured it in place.  The shower tray is now very stable without any popping or cracking noises.  It is also very lightweight!