[MRM]

I think you have a easy option that would look great without moving the footprint. Obviously, tear out the tub and put in a floor pan. On the right hand you have a low framed wall. Frame it all the way to the ceiling. Now you have the back and both sides enclosed. Do tile on the entire interior of the enclosure up to the ceiling. Enclose the front with glass and a glass door. I think that would look way better than any insert and should be able to use the existing lines.