A few years ago I made an interesting discovery / device.
I was interested in the exact shape of my bends so I got about thinking how I could map my colon. I decided to use a new dinner candle. First I made sure that I was fairly clean and pretty open with my double dong (its 18" long and 1.25" dia). After that I used the candle, I made sure that it could slide in fairly quickly without much resistance, but it was super thin too. When I could very easily insert it most of its length, 13" I'd say, I placed the candle in warm water until it got slightly soft and flexible, then slid it back in. The warmth felt amazing, but I let it just sit there for 10min to cool off and harden. I got up and walked around hoping everything would move back to its pre-play shape. Then I pushed it out; I'd say it had a slightly cork screw shape. I kept it for a few months and found that it could easily be 'spun' or 'twisted' inside very easily, especially comfortable as it past the sigmyd. I think it eventually broke, but the wick held the pieces together; this partly was why I ended up deciding the candle would be a safe(r) choice.
I would love to see a collection of these from several different people, I suspect that they would all have the same general shape but individually each would be unique.