I got some plywood at the local Menards store and cut them into 12X20 inch pieces. Some of them are fine but some of them are really warped! Here’s the solution I came up with.
I bought some small wall anchors, screws, and washers. I tapped them into the corner holes, put the wood down, then gently screwed the screws in to hold the wood down. They work great! I just leave them in but they can easily be moved to hold down any size of wood. Here’s a couple photos to show what I’m talking about.