Not only will that pallet wood notch out work great as a miter saw hold down stick, saving you $20.00 or more, but it is 100% wood, meaning that if you accidentally drop the spinning blade on it, the blade will cut through the wood - no damage to you or the blade.
Sometimes, with those plastic push and hold down sticks, the plastic will shatter into flying bits and could cause some kickback. Well, YouTube is full of those videos when you run a plastic push stick into the blade.