For Leveling Inserts
-Simply a 1-1/2" Oak Block, available at Lowes or Home Depot.
-Cut to length. I've found that I like them to be long enough to get 2 hands on.
-Dremel each end to the desired insert shape, and also Dremel the edges of the block to make it more comfortable in the hand.
-Sand to soften the edges, as needed.
I add heat with a hair dryer, to help the insert leveling process.
I use it directly on top of mylar, and have used with wax paper on CC playfields, though I'm not sure this is absolutely necessary.
The triangle-shaped one was created for use on the Funhouse clock Inserts, which are notorious for raising.