Here are .stl files for the tray.
This is a little different than the one i printed off; it's got a lid with through-holes to secure the lid to the tray with screws & nuts.
I haven't run this modified one off yet myself; still using the prototype tray.
This is made to use brass strips to make the battery cell contacts, and to make the connection to (and form) the battery contacts which connect to the internal laptop circuits.
My local hardware store sold brass strips in these sizes.
I had intended to use six off-the-shelf NiMH cells here, but the voltage was a little too high, so I use five, and span the empty battery location with the wires from a thermal fuse, and rest the fuse head between two cells.
I use the sliding battery cover from my original PB battery, but it's not connected to the printed battery tray; the tray has two spacers which contact the inside face of the sliding battery cover, holding it in place. These spacers are bevelled to provide fingerholds for removal of the battery.
This isn't the greatest - it's really skeletal so that I can print it off at the local library within the time limit given to users. If you're printing it at home, this might serve as a starting point; I think filling in the voids would make for a more robust battery tray.
PB TRAY.zip