MAKE a template for the plumbing wall (after the back panel is installed) out of scrap paneling or heavy cardboard - it's cheap insurance. After the template's holes are cut, position the side panel, making sure the top is level. Then transfer the shape of the back wall and the tub rim to the template with a scribing tool (such as a wood block and pencil) and finish shaping the template. The plumbing holes may need to be offset to allow for the material shifting after being scribed and cut.