I think you'd have trouble getting foam in there that didn't blow the slope and the bottom all out of shape. Those things are designed to fit on a flat floor, not a sloped existing shower pan, and that's part of the problem since the middle sits higher than designed. This is out there, but maybe some soupy plaster of paris? The best solution is tear it out, but that will cost some money. If you're willing to do it yourself, less than you think. Check out www.johnbridge.com.