Lots of thinking ahead. All electrical cables, for obvious reasons, had to be wired up to where they need to be, prior to panelling the ceiling and walls.
I've gone for 3 ceiling lights, with a light switch on the wall, a 12v point for a coolbox etc, and then a couple of USB points for charging devices. This is all wired up to a fuse box - each on their own different fuse rated circuit. Then to a leisure battery that's connected to the main battery by a voltage sensing relay.
The panelling is standard cheap tongue and grove. I've used thin and flimsy stuff, with the benefit that I'll be able to flex the wood into the position I want and bend it a bit. It's not loadbearing or structural, so doesn't need to be thick, plus it saves unnecessary weight.