Most good metal funnels these days have the "down spout" portion built in. If yours do, you don't need anything else. Simple use a hole saw to drill your hole in bottom center of your drum, insert funnel all the way through, screw or tack weld in place, and caulk the gap. Good to go.
If yours do not, you can simply use the small metal 2" "mini funnels" outside the hole of your drum. They are about 2" in diamter and about 2" deep. Screw to bottom of drum.