sorry, have no idea what sort of enclosure you have. Probably wouldn't know how anyway...
.....but please post some photos once you get in there....
I'm pretty sure mine has some little button cover things near the top that likely cover some screws...so I'm guessing that if I were to pull those then the panels would probably unclip from each other, &/or perhaps "lift and shift" our of the pan....maybe caulk holding around the edges too.
Regardless, I'm thinking it's probably held at the top and clipped in at the bottom and edges....
All just guesses.
I'd probably dive in gently pullin' and jiggling to hopefully figure it out.
or another thought. What's on the other side? My shower valves are in the side wall of the shower, which divides the bedroom, so there's some simple wall paneling that would be easy to pop off in the bedroom for a look-see from the other side.
Maybe if you were to post photos of what you have folks might be able to comment...or at least help brainstorm a bit better.