For the record, here's what I got/needed:
12x 1.2v AA NiMH Batteries - (20x
http://www.ebay.com/itm/271675325458
2x AA Battery Cases/Holders -
http://www.ebay.com/itm/170985400398
A NiMH Powerbook Battery [i don't know what the two internal components are, nor do I have the metallurgical skills to build the pins properly]
Solder
Soldering Iron
Krazy Glue
Tape
To open, I used one of those rubber mallets against a flathead screwdriver between the top and bottom part. It's just glued together to begin with and I think professional repack places may just use the mallet. I don't know which causes less damage to the case. I gutted the batteries and saved the pins and the back component. I cleaned everything.
Once you get the battery cases, on one end, the positive and negative leads go from different holders into the pins [that connect the battery to the computer - follow the +/- on the outside of the battery-pack case; red is generally positive for the holder wires]. On the other end, the other leads go to that [rectangular] component on the back of the original circuit - very important to note which side is + and which is -. Or I don't know if it's important because I don't know what it is; but I see no way of telling which was originally which otherwise.
I haven't finished this part yet, but I expect to tape around the battery holders (to keep the batteries in) and then put double-sided tape strips on the top and bottom of the battery-pack case to hold the holders in place. The wires I'm keeping between the holders to make the vertical fit better. Then I'll be Krazy Gluing along the edges and sticking it back together.