To use the flat lipo batts in this 9v batt box you have to do a bit of modding with both the batt and the case.
This is the battery I used for the build:
NEW 5x 3.7V 1200mAh 25C RC Lipo Battery Walkera 5#10 5G4Q3 SYMA S006 Helicopter
There were a few duds in my 2nd order from this seller that wouldn't hold a charge so they had to be replaced. All the batts in my 1st and 3rd order are aok so far. I've also bought the Storm 1200mAH 25c lipos which are the same size.
It's a tight fit height and width and depth. I had to remove the wrapping and de-solder the wires, then bend the tabs over to reduce the height. The width of the batt fits inside the case, but just barely. For the depth you need about 1mm more inside the case so I dremeled the inside of the front of the case to about 1.5mm in the area where the display is seated.
Retird is using E-Fly 1200mAh 20 C lipos and they are the same width and depth, but smaller in height so you don't have to putz with bending the tabs. Everyone is out of stock for the single cell, but you can buy the 11.1v set and de-solder...
Lipo battery AKKU 1200mah 11.1v 20C Max 25C JST E-FLY T551
The round lipos asdaq linked to breaktru used in his build where he put the fire button and display on the side of the case rather than the front.