Since you've already made a mod with an OKR-T/10, the box itself won't be a problem for you. You'll of course just run the positive and negative wires you would have attached to the batteries outside of the box.
So it comes down to a power supply. You have a few options. The easiest, (a laptop ac adapter with high enough amps, or a lab bench power supply) are prohibitively expensive, unless you have them laying around the house.
Other options are, a 12V power supply (made usually for LED strips) you can usually find them in the 10 amp range for around $20USD. Or my favorite option which is to convert an ATX power supply. Most folks have old ones sitting around, and if not you can find them cheap, new or used. The benefits of that, are a choice of voltages (you can run 12V for buck converters like the OKR, 5V for straight unregulated passthroughs, or 3.3V for boost boards.) You can run multiple passthroughs of different types at once from the same supply, and of course the price.
If you decide to go with an ATX conversion, there's articles and videos all over the place on how to do it. It's not difficult. If you have trouble finding them I can point you to some of my favorites.