This is a great forum and it’s a great place to get ideas and trade techniques. I’ve already learned a lot just by looking at and reading some great rebuilds thread. This is how I find the center of an area or surface. Also how I center whatever I’m installing in the center, in other words center on center. There may be a different way but this always works well for me.
First I measure the length and just divide the measurement by two. Then I make a few marks at the half way point, put a straightedge on it and mark a line across it. Then I just do the same thing for the width. Were the lines cross is the center.
Then I make an outline on a soft piece of cardboard or alike by putting something on the edge of “in this case hatch cover” and transfer it over to the cardboard by pressing down on it. You can use shoe polish paint whatever, I just used some wax I had laying around.
Then just make your cutout along the outside edge and you have your template. If the cover fits in to the cutout then you know it’s all good.
Then I just do the same thing I did with the surface that’s going to be cutout. Measure with and length of the template, make you lines with a straightedge and you now have that center. Put a hole in the center of the template were the X is and then center the hole of the X on the surface that’s going to be cutout and now you are center on center. Like I said there may be a different way but this always works well for me. . Pics are out of order but you get the idea.