cultured marble shower pan problem

I insalled a cultured marble shower pan replacing my tub. I placed it on a slab, but did not put plaster of paris on the slab to fill the arch that is built in to compensate for uneven slabs. I cant pull up pan because its fully installed. Is there any way to inject a material into the approx. 3/16 inch gap?

I had one that a homeowner had like that and the only thing I could think of was using a very long extention hose on a can of minimally expanding foam and drilling several holes behind the walls to get under the aprin of the shower. I had to use two cans I think. The customer was told this is not the right thing to do but he didn't want to pull up the base. I did try to think of a way to pump in structolite but didn't come up with anything.