Spread on a tray into a single layer: 175 degrees F, for about 15 minutes. Actually, I would imagine that you could use a higher temperature for about 10 minutes, but you would risk having the hulls char.

You could experiment with nuking which would be much faster; do a dozen seeds: start with 2 minutes on high, another dozen at 1 1/2 minutes, and another dozen at 1 minute. Hull a few nuked seeds and wrap in a moistened paper towel (marked with the nuking time) and place on top of the water heater or refrig. While there is some variance, most sunflowers will germinate in 7 to 10 days; no germination means no sprouting.

Frankly, roasting, even in large quantities, seems a waste of time and energy. It might also affect the keeping quality of the seeds. Why not have a "tray" made of framed screen wire (2 pieces, so it would fit around the post), which could catch the seed and hulls. Empty it every few days into the compost pile.