Here is what we ended up doing:
http://imgur.com/a/77KvI
The gearbox gear is put on a 5/16" rod with a cotter pin to hold it in place. That rod then goes inside a wooden 1/2" dowel which goes inside the black coupler. That way the 5/16" rod fits snuggly inside the wood and the black coupler. Last, a cotter pin is put through the black coupler so that it pierces the wood dowel and the 5/16" rod.
The whole thing was done with only a electric drill and a vice. And LOTS of measuring and precision work. So far it works great, but we are going to make a backup or two just in case.