I had two things: A broken penknife on my desk, and a need to showcase the new Silver PLA at http://diamondage.co.nz/pla.html - preferably with something shiny. Like that knife blade. Ahah! I now have a light handle that fills my hand well and has a nice lanyard hole too.
This handle is configured to accept a 2.5mm x 25mm x79mm blade with a 6mm pivot hole. The OpenSCAD file is parametric though, so you can fiddle around to suit whatever blade you happen to have at hand or knock up in the forge.
Make sure the plastic that your printer puts out is up to the job. In particular, don't skimp on plastic around the pivot or you'll give yourself a nasty injury.
Secure the blade with an M6 washer on one side, thin M6 press nut on the other, and low-profile M6 bolt to act as the pivot. There is no spring or lock.