Here is a 12mm Hex for an RC wheel, 7.5mm tall. Inspired by someone elses design which I found on thingiverse over a year ago but I can no longer find!!! =(.
I have it fitted on my Tamiya wild dagger, the inner hole diameter could perhaps being made bigger depending on your printer. I just used a reamer to get it to fit snuggly on the shaft.
Stl and Solidworks file are there to be used and modified as you like!