I would cut out the inside of the shape using a hand held jig saw. My problem in doing this is I get a less than straight edge for the bearing on the router bit to follow. I know there is a better technique than what I am doing. Welcome direction. thanks Brian
Make the template in multiple pieces, use lap joints to join the pieces together.
Since all you're after is the inside, the outside of the template can be any shape you need it to be in order to fasten all the pieces together.
"Life is what happens to you while you're busy making other plans." - John Lennon