Single construction shower/tubs are available with the clamshell top, but you won't be able to get one in an existing building due to door size, turn radius, etc. I have not seen one with an overhead, as most of the side walls are only about 6' from the floor level, leaving 2 feet of sheetrock. Now, as you are constructing this, and you change out the sheetrock for something more substantial such as tile backer board, you could apply ceramic tile above the shower unit, allowing cleaning of the shower, and wiping down of the ceramic tile. Of course if you are really ambitious, you could tile the ceiling and then the walls portions, making your own clamshell.