DIY portable phantom power supply?

[canidoit]

How hard would it be to build one?

Does anyone know of any kits around?

 

[zebra50]

Just stack five 9V batteries to give (a little over) 45V.

You'll still need the matched 6K81 resistors, and some blocking caps if your preamp doesn't like DC at the input.

Stewart

 

[RuudNL]

Five 9 volts batteries is of course a possibility, but I think it is not the most elegant solution.
Personally, I would choose a step-up converter and use a lower battery voltage.