hi. Love to help. First, I would not cut it as big as I did. So to help you out right, let me say that these measurements are to where the hook is not where I cut. Here are the measurements as you asked for them. The X is 10.5 Inches from the outer edge of the plastic and the Y is 4.5 inches from the same edge.
But you will see that I did not go 10.5 for my first cut. I made my first cut (in the picture you used) on the right at the point of the plastic stopped so as not to cut in to it. I would start there again. From that point I went over 6 inches for my sec cut. WAY to wide.
If I where to do it again, I would cut as close to where the hook was going to sit (measurement for Y) and only open that up if you are not happy with how tight it is. Also The over all height of the opening is 1.5 inches currently. I would have started at 1 inch from the bottom.
Second, If when installing the hook, you will remove the top mounting bracket and install it separate this will go easier and you can keep the hole smaller than I did. coming at the opening from an angle then straightening out after it is through the hole.
Really hope this helps. Sorry it is so long.