At pesent I have a free-standing shower (touching only the back wall). I want to replace with a nicer shower(one that is not free-standing - touching 3 walls). I will not be able to find a shower that is exact size between the present side walls. I will need to extend the two side walls out about 3 to 4 inches to meet the shower walls. What is easiest type of wall to build? Do the shower walls have to be glued to drywall or will plywood work? I am limited on placement of shower due to the drain which is exatly 18 inched from the back wall.