Hi,
If the rod holder is on a 30 degree angle,
then the hole will have to be more like an oval
shape. Get a piece of scrap ply wood or something
and do a test hole first, that way you can see if
the rod holder sits nicely in the hole.
Then you can use it as a template.
If you dont want to spend the money on a
hole saw, just mark out the hole and drill a series
of holes around the marked area and punch it out