Do you have any photos you can post? I'm assuming the enclosure is plastic and if that's the case you may be able to heat and reshape it, but it'll likely be ugly.
I'd be a little weary of using individual sheets for the enclosure instead of a 1 piece surround with all the movement. Do you know if the pan extends to the walls or will you still need to pack it out to install new walls? You may still be losing some space if it doesn't.
Have you looked at any replacement pans and surrounds that may be a better fit? Do you know the dimensions of the space you're working with?
I'm thinking about doing a custom gelcoat fiberglass wet bath enclosure in our new project trailer, but haven't decided on that yet.
