I just got parts for my Power Half from Sparkfun. I'll be using 2000 mAh 3,7 V LiPo as a external battery.

Power Cell will be used for charging the Power Half and converting the 3,7 to 5 V. I'll take the JST connector off and solder the battery straigth to the Power Cell. It's possible to add a SPDT switch into the Power Cell to control its state. Total cost for this set is about 30 eur.

There's microusb port in the Power Cell so the Power Half will be charged via it.

I have also some 5 mm Copper Tape to connect the power into Jolla.

Thickness increase will be about 5.5mm. LiPo at the left side in the picture is 1200 mAh.

I don't have enough time this week for prototyping (except taking pics of the parts ) so you have to wait for next week