I've found it much easier to get a smooth application of both the epoxy and varnish if the hatch opening is cut after all the other work has been finished.  I use my jig saw to do it, with a metal-cutting blade to ensure a mostly chip-free cut.  I also put blue tape on the bottom of the jigsaw to keep it from scratching the finished deck, as well as more blue tape on the deck itself, for the same reason.  Then I wipe a quick coat of epoxy on the cut deck edge to seal it, and install my deck plate.

   I have cut after on both our kaholos but instead of a jig saw I only had a dremel with a cut off drill bit and attached to it and it has worked great for me. Just had to cut in quarters leaving a small tab to be cut after withouth the weight of the device. 

https://www.google.com/search?q=dremel+circle+cutter&client=safari&hl=en&prmd=svin&source=lnms&tbm=isch&sa=X&ved=0ahUKEwig74PJ2OjPAhWoqlQKHYE7AokQ_AUICSgD&biw=768&bih=928#imgrc=tTjxFMkJ2LBUlM%3A