After two trips to Home Depot and one trip to Lowes, the coupler that fits is a 3″ PVC, it can be found in the sewer and drain fittings section. The schedule 40 and schedule 80 fittings are to thick to fit in the holder. Black ABS fittings are just a hair to small. I also tried some reducers, that are wider at the top but found they fit a little loose in the cup holder, the straight PVC coupler friction fits very well.Thanks for the idea!Last edited by Tnbigdaddy on January 29th, 2011, 1:52 am, edited 1 time in total.