You can make all of the slates match by finding someone in your area with a 36" wide wide-belt sander. Run the thicker slates through the wide-belt, slate side down, sanding the wood backer until it is the same size as the smaller one and then run all 3 pieces to make them exactly the same. I do this every time I install new wooden backers on a set of slates, works fantastic. A 36" wide-belt sander is a fairly common tool in professional wood shops and a lot of them will rent time. If you are lucky enough to find a couple in your area go with the one that has a Timesaver or SCMI unit.