FOR THE DOWN PIPE CONCEPT, WE BELIEVE THAT CARDBOARD POSTAL TUBES WOULD BE SUIT THE FUNCTION WE REQUIRE. THE AUSTRALIAN POST WEBSITE LISTS THREE SIZES OF TUBES AS DEPICTED ABOVE. TO BEST REPLICATE A DOWN PIPES, VISUALLY AND IN TERMS OF FUNCTION, MAILING TUBE Tu2 OR MAILING TUBE Tu3 WOULD BE MOST SUITABLE FOR THIS APPLICATION.