The best way to do it is buy a long 4xAA battery holder (2x2 batteries, lengthways) from an electronics shop / website and transfer the old battery pack connector onto it. You don't even need to solder the wires, it is possible to use butt connectors or even simply twist them together and tape them. Then you can put NiMH AAs in and remove / change them whenever you wish. Much easier than fiddling about with soldering batteries together.

Note: you will probably need to lengthen the eMate's battery compartment a little by trimming off one of the plastic ends. It's pretty easy to do and doesn't affect the eMate's appearance at all, since it is an internal modification.

Here's a link to a pic of this setup that I made on my eMate:



With the 2700MAh cells I installed, my eMate can run for 30+ hours on a single charge!