Good that you're thinking through this part of the design. It's worth the planning to get it to work for your use.
I'm converting a shuttle bus right now and have a 32x40 space for a shower. I plan to build a fiberglass pan and I like your idea of wood grate floor on top of it.
The side walls are 3/4" plywood, stained and sealed. The back wall is the bus outside wall and I'll have a half-wall of 3/4" plywood up that. I plan to hang a custom shower curtain that drapes inside those plywood walls. No pictures to share yet.
Probably the biggest design constraint to consider is how often you plan to use the shower. If you are planning on extended use or full time use, completely watertight would be the right way to go. Consider Home Depot waterproof vinyl paneling, or if you're truly ambitious tile all the way up to the ceiling. Since we are designing our bus for max 4 days off grid at a time we're doing a less rigorous approach.
My composting toilet is on heavy duty drawer slides on the left side that open into the shower area (that will be the most frequent use for the shower area). I haven't finalized the watertight drawer seal design yet. On the right is the access panel to the propane closet.
The front of the shower unit will be a slider that can extend into the living area about 8 inches, making the shower space big and roomy (though you won't need to slide it out to use the toilet or shower).