I am seriously bothered from lacking voltmeters in blasters, especially in blasters with low capacity batteries like the nightingale.

I couldnt find a file for a frame and didnt want to cut the shell so i modeled a new battery door with a bit more space inside for slightly longer batteries and a slot for a voltmeter. 
the voltmeter will friction fit in there and if you add the frame you wont push it in on accident but you can use short m2 bolts to fasten it in there if you need to, be careful with the pcb on the back of the meter though. 

For wiring i put the black wire into the top of the revswitch and the red part of the flywheel cage wiring so it just turns on when the switch in the back is on. this way you also wont need extra cable and you just need a soldering iron and a speck of solder. 

Refer to the picture for help. (note: im not an electrician and dont know much about it, it just works for me. Please be careful with lipos)