It doesn’t even need to be plastic. The battery could slot into the side of the phone on a tray with a gasket to try and seal it.

Personally I don’t think the battery needs to be as easily replaced if it lasts longer. Lithium ion cells degrade too quickly but a lithium iron phosphate cold last for 10 years before dropping below 85% charge capacity.

The only drawback is they have about 30% less energy density but imo making the battery 30% larger is not a big deal. Phones have obsessed with being pointlessly thin for so long. Basically just remove the dumb camera bump.