I have several halogen lamp driver PCB's I have done at work. Some of them run 25 or more 12 volt, 20 Watt lamps. I just have the soldermask omitted on the drain and source traces for the FET's, make the traces as large as possible, then reflow them with solder during assembly. You can get really large densities and circular mil areas on tracks well reflowed.
I have 50% 1sec on -1 sec off duty cycles at 500 Watts minimum on these boards, no problem. Figure the inrush to the halogen lamps, and the average current is very high. Barely perceptable heatrise on the traces.
Position FETs in a row, do the D trace topside, go wide as posible enveloping the gate, do the S trace bottom side, and spread outward, wide as possible, gate bottomside.
I only use 2 oz, reflowed.