Some baking tips.
I just went on Amazon to look up baking sheets and pan extenders. Not all 1/2 pan baking sheets are created with the same dimensions. Nordicware is one of my favorites. I am not sure but I think all pan extenders are the same dimensions. If you go to a restaurant supply store you will find 1/2 sheet pans in the correct sizes. If you go to Home Goods you will find a variety of sizes that differ by inches all called 1/2 sized sheet pans or sometimes cookie sheets. Buyer be aware!