I've used cheap, construction grade 1/2" plywood, as well as 1/2" mdf. The Baltic birch plywood is probably the best material, but if you don't have it, the other works just fine. I also found that putting a coating of paste wax on the insert helped the wood move across it.
When I made mine, the insert was a bit sloppy fitting. I wrapped tape around the outside of the template, until it fit tighter.
Sometimes I wonder whether the world is being run by smart people who are putting us on
or by imbeciles who really mean it.