Libra Replacement Battery for Kobo Libra H2O

[ovalwonder]

I have a Kobo Libra H2O that has suddenly decided it doesn't have a battery (only runs when plugged in, always shows 100%). I know that there is not an official replacement battery, but is there anyone who can point me to a good replacement (or at least the appropriate size specifications)? I'm borrowing my wife's old Kindle Keyboard, so I'm willing to wait on a replacement to ship from wherever necessary, and am perfectly comfortable with a soldering iron, but don't want to spend a ton of time and money waiting on incorrect batteries.