I made one by setting my insert on a piece of 3/4" MDF and surrounding it with strips of 3/4" MDF that were stuck down with carpet tape. Then used the same bit I'd be using to route the insert. Remove your insert and set it aside, route inside the sticks with appropriate sized flush trim bit. For the template, obviously, you're going all the way through. When you route your actually insert opening, you only go the depth of your insert.
Once you get all the way through the template piece, don't throw away the piece left over from the middle. Trim off about 1/4" on all sides of that middle piece (this doesn't have to be real accurate) and when you have your template on the table and positioned where you want it and stuck down.... then take that middle piece and stick it smack dab in the middle of your template opening. Now you have better support for the router when routing for the insert.
I'm sure there's lots of ways to do this. I've seen it done with template guides too.