better to have a joiner make a new bath panel to size in say MDF or marine ply. my new steel bath replaced an older cast iron one which was much deeper so to avoid having to completely re-tile the walls, our plumber set it on a plinth, the bath panel which came with it was thus not tall enough, our joiner made a panel to fit the space, including a removal end hatch for easy access to taps/drain for maintenance. He used MDF and sealed edges with waterproof sealant. we painted it with water resistant paint. was in place in good condition for over 10 years before we moved.