I took a trip to home depot to get some odd and ends to serve as starters for a mold should I try one.
I discovered that 4" sewer pipe is s bit under 4.2" in diameter.
Enough for 5 models cost $5.
I also found some couplings that are spot on for the 3rd stage. I'll have to glue them together in sequence and they're a bit more expensive at $1 each. A few in sequence should do the trick.
All told, I spend under $10.