I think the concept of the DKLM gearbox is pretty ideal. The price is a little high but it is maintenance-free and universal.
All you need to buy is a new mount to share it on another tank - you can simply remove the gearbox by turning only 2 screws (without tool with wingnut mod). It is a smooth slide-fit.

The gearbox is also available in 3 speed options (slower = more torque, but even the fastest one is more than sufficient for tanks <20lb). I have pre-ordered a fast-gear one for my Leo1.