Use 2" PVC pipe with a 2" NPT fitting solvent welded to the end of the pipe. Use hole saw to make an opening in the seat base on the side of the seat base that faces the wall. Silicone around the opening and use a jamb nut to keep it in place. Use silicone to plug all other openings. Attach a plywood cover over the rear opening and use silicone to seal it in place.
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