I am replacing an old shower pan with a fiberglass unit. I believe the old pan is acrylic. There is a large opening in the slab for the trap, approx 12" x 12". There will be no support for the part of the pan that's directly above this cutout. Is the fiberglass shower pan strong enough to not need additional support in that area without cracking? Or do I need to do something to provide better support for the pan in that area?

Thank you!

Looks more like a tup dap out to me then a shower installation. When we rough for a shower installation we position the trap, bring the raiser from the trap up out of the floor 2 or 3 inches and then pour the floor leaving the trap under the pour. The only time we leave a square hole is when we rough for a tub. If you leave the dapout and install a shower base over it you will have no support for the shower drain. The floor will flex and give with the weight and eventually the seal between the drain lip and the shower floor will be ruptured and start to leak. My advice? Extend the raiser up over the floor line and fill the dapout with cement. But you're not through yet. Most plastic or fiberglass shower basis need to be bedded so the floor won't flex or give when you step in it. To bed a shower floor, puddle cement or mortar under the floor after it's been installed and let it cure. You now have a shower floor that will last without leaks. Good luck, Tom