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The centre terminal is the common ground or Negative for all bulbs. The copper wire I used was stiff enough to hold the bulbs upright.

I made a face plate to cover the cases power connector holes. I'll fit two switches here, each switch will switch in 2 bulbs, or 100 watts. Below you can see the completed unit, minus the lid. The bulbs sit about 15mm down from the lid.

Below is the circuit diagram. I've divided the dump load into two 100 watt switch able loads. The two switches each feed two bulbs. I need the fan to be running when either load is switch on, so I used a couple of diodes ( 1N4001 ), one from each switch.

All finished and running. Looks good eh? At full dump(?) its dissipating 200 watts. The sides get very warm, the top gets hot, I measured 85⁰C ( 180^oF ), so don't put anything on top, including little fingers.

In future I might add a heat sink to the top of the case, just to dissipate some of the surface heat.