I just use 1/2" cabinet grade ply.
I first put in leveling screws from the bottom, but discovered it was much easier to level, if I drilled thru holes in the corners, and tapped for socket head set screws. This makes leveling from the top easy, without having to remove the insert to adjust a screw.
With the right size holes, screw can just be screwed in, without tapping the threads.
I also use Bondo, to fill a worn slot in the inserts, before cutting a new slot.
Make at least 2 inserts. Keep one with no slot, so if you need a fresh one for a job, it's ready. Or if the one being used gets wallowed out, just swap for the fresh one, and bondo the old one when you get a chance, and have it ready to go.
Don't forget to have a few extras for different width dados!
A while ago I got fancy and made an insert with an insert. Stopped using it. It was easier to bondo slots, than make a new insert for the insert. But it was fun making!