Hi, just dropped by and can help you out a bit. I just measured my Countryman B3 mics power voltage at the output of its hardwired XLR phantom power adapter as plugged into my Sound Devices 702 recorder's phantom powered mic inputs. The voltage I measured that is going to the B3 mic capsules directly is 11 to 12 VDC. The red wire is positive and the shield is negative polarity setting.
So that's all you need to know to get started building your own battery box for them. I think you can use the standard 9V battery box schematic to power them and they will work fine. They may not have as much voltage presented to them as the 11v to 12v output by their XLR adapaters, but I think you can use them or look for a small higher voltage battery than 9v to use in your battery box.