Hi I would ensure a water
tight "tank", so I would seal it which epoxy
, you do need to remove any flexing ( as it any obvious flexing), seal it with glass cloth and epoxy or vinylester resin. You could even gelcoat
it ( with a wax additive) or just add a surface capable of handing the water. I used white stripped Teek-Dek in my case. ( Though either seal it completely down , ie waterproof, or traet it like a removable mat).
IN the second one I did, I actually moulded a custom fibreglass floor for the head/shower, Thats proved to be the best solution, I built in a "lip" so that the ply shower/head side panels
ended flush etc. Its essentially a "tank"
You do want to ensure no leaks
as shower water over time does lots of damage and stinks the place up.