An adaptation of alany's tube key http://www.thingiverse.com/thing:21410
that includes a squasher frame and end cap to hold the key in place. The slot in the squasher is offset to help the tube go around the key.

Updated June 2019

Anything marked v2 is the updated version. You'll still need the original key part.

The cog is slightly larger, giving it a larger hole. As a result it is a loose fit on the key, which makes it easier to remove when you want to take an empty tube out. In use, the ratchet holds it in place, but it you want to make it permanent, a bit of super glue (CA) will lock it to the key.

V2 of the frame has a beefier ratchet spring (it's still a thin bit of PLA, so it will always be delicate). The new frame also has a bulge that pushes into the neck of the tube in an attempt to extract every last bit. But as Bill Bryson once observed, there's always more toothpaste in the tube.