For a very quick drive system with wheels mounted to the 3/8" unf thread that comes out the gearbox...

I melt these:
http://www.ebay.co.uk/ulk/itm/191818584715

into these wheels:
http://www.ebay.co.uk/ulk/itm/270510822899

then put the reverse threaded bolt in the other side. Works awesome.
can even put a 3/8 unf bolt out the other side to go into a bush or bearing so there is no side loading on gearbox if you want.