Foam cutter template material!
I found a great source for aluminum template material. There are probably cheaper alternatives but this is a reliable source that is open late. I use aluminum kickplate from Lowes or Home Depot. This is the stuff you screw to a door to protect the bottom from scuffing. They have brass too but it costs twice as much. The aluminum cuts nice with a saber saw using a fine tooth metal blade. I saw close to the line and finish to the line with a 3" dia sanding disk mounted in my electric drill.
Rob