method for cutting hatches?

Of the many available methods, this is mine, carefully honed over a grand total of six hatches:

Using a full-size template (or half of one which gets flipped over) scribe or scratch a nice clear line. Get a nice, bright lamp nearby as well as a box fan to blow stuff away. Use a 1/16" drill to make a close series of holes, nearly touching each other, about 1/4"long, i.e., the same length as the depth of a jigsaw blade, all touching either one side or the other of the scribed line. You can further connect them by wiggling the drill bit along the dotted line, but don't try to make the full cut with the drill bit as it'll be wobbly.

Mount a fairly finetooth blade in the hand set with the cutting on the down stroke. This way any splintering is on the inside, though if you scribe deeply enough you might reverse this. With a little back-and-forth you'll be able to work the blade across the little holes and penetrate the deck.

While cutting, to prevent coming all the way out and then punching a small hole in the deck about half an inch away (how distressful!), I clamp on a small pair of vise grips on the inside. If this is a CLC boat, and that bulkhead is already there, you're out of luck on that one. Maybe you can grind that end down to a rounded tip that will just dent the deck instead of puncture it.

Take your time, stay on whatever side of the line you started with, apply gentle pressure, let the blade do the work. Every 6" or so, stick some tape across what you've just cut, so the hatch won't want to fall in and tear up the final inch of cut.

Small wobbles will disappear when you do the touch up sanding.

Good luck.