I really liked the original but could not get the axles to print at proper size after laying the original model down on the print bed. (If you do this, take care to put it on the correct side!) My solution was to print a spacer that takes an M3 bolt and assemble using two M3x12 bolts and hex nuts.
The spacer's outer diameter is just enough smaller than the hole in the gears to allow them to turn freely. The bushing is just slightly (about 0.5mm) longer than the thickness of the gears. Depending on your printing conditions you may need to adjust the length slightly. The idea is to leave just enough space for the gears to turn freely, but not so much that they can wobble.
Also take care when you tighten the bolts that you get them just snug enough to stay put but not enough to compress the spacers and keep the gears from turning.
3D Hero PLA