might suggest, a few shower heads.
bathtub/shower combo i have, i have the the generic shower head, with extra shower head on a hose. over the years, it would have been nice to have a shower head on a hose on both sides. so i could leave the shower curtain open, and then prop myself against wall, and use one shower head, and then turn around and use other shower head, without getting tangled up in the hose and reaching extra length for the shower head on a hose.
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instead of building an actual stall. perhaps turn to more of a "large shower area" that includes toilet, sink, and a various handle bars. so you can physically shower while seating on toilet if need be, and just let the water from shower head go directly down onto the floor. and drain down a couple different drains. 1 primary drain for general shower area, but another centralized drain further out.
if one of you needs to help other shower, being able to seat them out in the open, were you can physically walk around them yourself. without slipping and falling and squeezing into odd positions trying to get around them to wash them.
and yes the sink is included in the above. so you physically reach over to a sink of warm water, and use a cup or bowl and grab water to rinse someone off. vs using shower head or what not. having it all incorporated so were ever water falls it will go down a drain vs flooding your home.
how many times have you washed your face / shaved or what not. and used a wet rag to wipe off. vs perhaps wanting to use a cup of water to rinse off but could not type of thing. ya it be messy due to no drain. but ya gotta do what ya gotta do.
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there are different types of connections that can be made between "liner" and drain lines / water lines coming up through the floor to make them water tight.
google
pipe boot site:koiphen.com
might bring back a few options.
there is also various types of flanges that can be used to help create a water tight seal. including just running 1.5" to 2" shower drain right through the liner, and then running your cold/hot water lines through the shower drain. nothing actually connected to the shower drain, just using the physical fitting for a water tight seal.
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a shower liner, can be epdm pond liner, to flat roof liner. to other. there are better "folds" for in the corners.
tile and groat it all up. (floor and walls)