I've also taken the advice of a manufacture to use spray foam. WHAT A MISTAKE. I set a 200lb cultured marble bathtub with it. I placed 3-400 lbs of tools in the tub after I sprayed the foam. The next day, even with all the weight, the tub had risen 2" off the floor from the foam expanding underneath. You don't want a 1/2" thick bed underneath the pan, just something to stabilize it. Thinset mortar is the way to go. If you are concerned about not filling the voids in the pan, flip it over and fill them with thinset, let them dry, and then set the pan in a thinset mortar bed. Cement and the pan will not react, don't use plastic on the pan side. I use tyvek for the floor only because it's clean. Staple the snot out of it when you put it on the floor. 1/8-1/4" of mortar is all that's needed for a good set, the pan is essentially a large tile.