Tape off the deck in the area you are putting the deck plate. Draw the correct circle and use a drill/ Jig saw (fine blade cuts nice).

No need for a 50 hole saw you will neve use again. 

Some folks just use a Bonsai saw if you have one of those.

David

If you have a dremel tool you can use the 678-01 Circle Cutter attachment about $12.

I used a Fine blade on my jigsaw. I cut inside the line, then cleaned up & enlarged the hole with coarse sandpaper. You could also try a coping saw.

I used a jig saw and taped off the deck and it worked just fine.   Was scary cutting into my freshly completed Kaholo but it was really easy.   No need to buy anything expensive for this.

Draw a circle with a compass, drill a small hole inside the circle and use a router with a small bit.