Stubby VexIQ Motor (square shaft) to LEGO Technic Axle (cross-shaped).
This is a stubborn print: even on the MakerBot Method there's a tiny bit of sanding to remove the elephant foot at the base. You'll almost certainly need a raft. This version prints with no supports.
The axle fits fairly snugly on the Method prints; you might need to scale it up/down slightly to match your printer's accuracy--the square shaft can be sanded, the axle (cross) insert is essentially impossible to modify, so size to that, and sand the VexIQ side as needed.
Converter in action here: https://youtu.be/zAYUtxhk-GM
(We'll be doing a servo version next, because our LEGO Forma is going to be hooked up to the interwebs!)