From the "Don't try this at home kids" department....

My daughter has an orthodontic expander but lost the key yesterday. The orthodontist is closed for the week and she is supposed to have it cranked every day. I happened to have some nice stainless steel hex keys and for my first prototype bent one at an angle (see photo) and it was successful. Then using FreeCAD I designed a more elaborate thing as shown here.

The blue and white tool in the photo is what I am replacing with a 3D printed version. After it's printed I will cut down one of the hex keys and press fit it into the hub.

Edit: As soon as the glue dries, I can snap the yellow pieces together, and it will get used for at least a few days until the next orthodontist appointment.

Edit 27FEB2017:

I had a successful use of the home printed version on Saturday, then on Sunday my daughter found her original key so we went back to the OEM version. It was interesting to compare mine as printed with the OEM version- one feature lacking in mine was a little jog in the business end that keeps it from going in too deep and potentially scraping the roof of her mouth. Dimensionally otherwise my version was pretty close to the OEM version. Verdict: successful experiment.