Add support to existing acrylic shower unit

When we had acrylic shower units installed in our house apparently our employee did not know that they require concrete support under their floors. We learned about this the hard way when one of them broke with use, and we were obliged to replace it. We've used the others only infrequently since making this discovery.

Obviously it would be difficult to put concrete between an installed unit and subflooring now. Additionally, a warranty claim is not possible due the installer being under our employ (we paid him hourly), this problem is now ours to fix. Is there a product that could be used that would provide sufficient support? I assume we need something that would expand slightly to completely fill the gap with exerting too much pressure on the acrylic shower unit.

Thanks for your answers or suggestions!