I also thought about a battery mod.
Instead of searching for a battery with the same dimensions, I was wondering if it would be possible to install a second one with the same size and connect them in series.
Currently my Hi8 is placed in this case
http://www.banggood.com/Tri-fold-Stand-PU-Leather-Case-Cover-for-Chuwi-Hi8-Tablet-p-987129.html
Given the hight of the installed battery, 3mm, I think it could fit in the case. Removing the backcover of the tablet + cutting out some plastics within the case should provide enough space.
The lack of electrical engeneering know-how prevented me from realization.