I followed up your great work.
(used motoroi X720 Battery that I doesn't use it any more.
And nspire Touchpad)
First,
Tested it briefly to know whether it works or not.

It worked Perfectly!
Second,
Assembled battery like below

(soldered nothing.)

Battery pack is little bit thicker than official one.
So there's a gap in back panel.
Anyway it was powered on well.
But couldn't check the battery status.

(Of course AAA batteries was removed already)
Maybe should I solder two sets of pins (++ / --) together to solve this problem?
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[Added]
Status problem Fixed

P.S) It seems it doesn't get charged through USB connection