This is indeed perfectaly doable if you have a little experience in electronics and soldering. You need two closely matched 6k8 resistors per channel (1% types is preferred, but you can hand match). Connect battery ground to pin 1 of the XLR. Each pin 2 and 3 gets connected to plus through the resistor. I would add some high frequency decoupling at the battery side (a small condensor).
If your recorder can take 48V on its input this is all you need. If not certain, you need to decouple the 48V from the output side of your battery box through some largish condensors that can stand 48V. I would add some overvoltage protection as well (diodes) to protect the recorder.
Gunnar