Squeaky shower insert

I have a fiberglass shower insert in my upstairs bathroom. Over the last couple of years, the floor has been squeaking more and more. What can I do to stop the squeaking?

If you can slide some sort of support between the fiberglass unit and the subfloor, that would lesson, if not, stop the squeaking. The best material would be a mortar or cement.

I don't want to suggest expanding foam unless the base is secured to the floor with no chance of moving. Expanding foam may create other issues.