I decided to persevere with the 24v supply and dug up another globe and added in parralell the 0.6ohm load resistor bank from my astratech battery tester.
This allowed allowed around 25A of current to flow half way to my goal of 50A.
But I still suffered a failure of the main FET. The device is supposedly rated at 100A continuous which should be adequate for the job. Perhaps the instantaneous startup currents that my test fixture will allow are in excess of the maximum that the FET will cope with.
In both cases when the failure as occurred the FET has shorted from Drain to Source.
In my circuit I have allowed for 2 FETS to be connected in parallel but have only been using one perhaps a second FET is required to deal with the instantaneous currents.
I must check the turn on current of the halogen globes.
Measuring the cold resistance of the two globes I'm using in parallel is around 0.1ohm 12/0.1 = 120A plus the 20A from the resistive load.
That's 140 A inrush current so it looks like my test fixture is the caus eof the destruction.