I took a trip to the local Home Depot last night to have a look. I found that the 4" caps intended for perforated drainage (sewer) line just fit the 4.5" wide deli cups. The 4" couplers worked also, but they're too tall to fit well inside my 28 quart tubs. This will work great for my larger snakes and tubs. I'm going to try to find some smaller cups and pipe to fit in my 6 quart and 12 quart tubs.

Steve