I discovered that the less air draft inside the 3D printer, the more smoothly the 3D print relaxes into it's permanent shape. This means less likelihood of warp.
This thing is designed to snap into place in the upper hole of the lid. I've included a 1:1 scan of the lid so people can make their own model if they so desire.
This is made to be compatible with http://www.thingiverse.com/thing:1672513
I've used it successfully so I've taken off "work in progress."
Has 30° overhang for the snap-in-place parts. ABS for strength.
I filed the overhangs to make it fit a bit smoother.