Shower Drain Void-How to support Schluter pan
I removed the original builder's fiberglass shower insert. The concrete foundation has a 1' x 1' void area for the shower drain and P-trap (see attached photos). I've self-leveled the concrete and plan to install a Schluter drain pan on the concrete foundation. My concern is the foam pan not holding up to the weight in the void area and eventually collapses after continued use. What is the best option to ensure a solid foundation underneath the foam pan? I don't want to fill the void with concrete as over time the concrete would probably break the plastic with expansion and shrinkage due to weather changes. I've thought about filling the void with sand but not sure how I would adhere the foam pan over the sand area. I've also considered filling the void with spray foam, but again, would have the same issue adhering the Schluter pan to the foam. Another option might be to fill with sand and cover the sand with a couple inches dry pack, but I'm not sure if that would truly hold or just crack up. Any suggestions or help would be greatly appreciated. Thank you!
Appreciate the advice. Figured I’d follow up with a brief description and pics in case anyone else encounters the same issue. I knocked away the plastic, wrapped the drain pipe with foam tape, loaded the void with gravel and sand, and poured 4” of concrete on top. I placed a larger pipe around the drain pipe coming up to avoid it being completely surrounded by concrete. I ended up having to cut about 1” off the top of the surrounding pipe to enable the FloFX drain to fit properly. Schluter shower pan and FloFX drain installed. Water test in progress😁