I am replacing acrylic garden tub with 60 x 36 shower. I bought an enameled cast-iron base. I am tiling the walls. The shower base is exactly 60" wide. The shower space (2 x 4 to 2 x 4) is 60 3/8" wide. 1/2" backer board will be used as the start for the shower walls. It runs into 1/2" drywall. My problem is the extra 3/8" space on one-side. I do not want to rip the drywall out and run a 3/8" filler along the whole wall. Is there another way to handle this?