So I've been using my old mini t-driver for a while now - http://www.thingiverse.com/thing:65138 - but I wanted something a little beefier as I found the old design tended to crack after a while. The source design is available from Onshape - which as a bonus contains an alternative design more similar to the original with much large shoulder chamfers. https://cad.onshape.com/documents/91d9e7bf22737ed31f8afdae/w/82338bd6e8bebd14c6d7abcc/e/0abfbe9a75ca5a84fc3b2861