Add support to existing acrylic shower unit
The problem with expanding foam products is that you cannot effectively "control" the expansion, so it may expand in the gap to the point of damaging your acrylic from the other direction! The type that would be less likely to do that will also not really provide meaningful support; it will just compress with the flexing of the acrylic and you are back to square one. 

I think a warranty claim against the installer is your best option. He did it wrong, he should make it right. He will not like it though because it will likely be impossible to remove the shower units without damaging them, they usually adhere them to the walls with construction adhesive.