Mounted my auxiliary battery under the seat too. I just used a surplus battery tray from my race car. Ran the cables up to under the hood. Used a kitchen plastic cutting board as an electrical board for a marine breaker module, buss bars, relays, remote charging posts & switch panel control box. Also mounted the pistol grip winch controller receptacle.
i ran the roof light bar wire, rear flood light & switch panel cable up the roll bar tube, drilled an exit hole at the crossbar junction facilitating a complete disappearance of all the wires. My roof mount light bar is hanging under the roof cover suspended by the crossbar. The switch panel is also attached to the crossbar keeping both units completely out of the weather. A headlamp elastic band wrapped around the crossbar makes a great Dome Light.
it has all worked out well thus far.