Here's the power installation, and for the final cut I've made a facsimile of a Fullymax 12S pack of 5600 mAh that is normally used for RC helis, but which we may just get away with here. It is the type that includes a pair of six-cell packs sealed end to end, so that the cables would appear from the right in the picture.
From thence they power the ESC, which in this case is a facsimile of the Flame 180A that we've used before in test-flying. From the right-most end of the ESC, therefore, the batteries connect to the underslung motor.
Am pleased with the (wooden) mock-up battery-packs, as they represent a first foray into shrink-wrapping.