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.
Push the key through the two holes - it's meant to be tight - and then push the end cap on the key to stop it coming back out.
It was pointed out that the .stl file had two parts higher than the third one, so I've tried to realign them and centre them on the platform. New .stl uploaded that is, in all other respects, the same as the original.
I beefed up the frame a bit as some people said it was too flimsy. I had also noticed that, whilst aluminium tubes work ok, plastic ones try to unroll themselves, so I turned the end cap into a ratchet wheel and added a spring to the end of the frame. It seems ok, but only time will tell.
Use the original key, but use the tpr2 ratchet and tpr2frame to make the improved version.