to attach the PVC to the 2x4, bore a small hole in the top of the pvc, just big enough so you can get the screw and screwdriver through. dont worry, it wont affect the sliding surface at all if you do it right. as far as stabilizing the whole thing, just build some good supports... when i built my pvc rail i used 4x4s as the vertical supports and 2x4s on the base as stabilizers. i think i also used two short lengths of 2x4 on either side of the vertical 4x4, sticking up a bit higher than the top of the 4x4, so i could just line up the two supports and then align the 2x4 with the PVC pipes (two abreast) between them. worked like a charm.