If it's just for charging current
(not motor current) then you will only need ~3A current carrying capability, but if it's for running the motor, then it will need to carry a much larger current.
The following connector is probably a bit too large for just charging use, but it is still an option:
Click here for more details.However, if you were actually planning to power a Magic Pie though it, the connector should be rated for at least 25A and ideally 30+ Amps.
You could even use extra connector pins to share the current load, i.e. a 4, 6, or 8 pin connector could be used instead of a 2 pin connector using 2, 3, or 4 pins for each of the two battery cables.
If the connector pins individual current rating was 10A you could use a 6 pin connector to carry 30 Amps between 3 pairs of pins wired in parallel.
So, something like the above 30A 4 pin connector could probably be used, combining 2 pins for the positive lead and the other 2 pins for the negative lead.
As I'm guessing the 30A rating for the above connector relates to the whole connector - not each of its pins.
For high current power connections, most people tend to use the large inline anderson powerpole connectors, and these seem to work fine even though they are not waterproof.
Click here for more details.
Alan