I have done a lot of this kind of work using a Bosh saber saw with a fine tooth wood cutting blade it makes few if any splinters. An expensive but excellent tool. Makes cutting curves vey easy. For a cleaner look i finish both sides of the opening with a router using a bit with a radius of 1/2 of the thickness of the plywood. A little sanding with 120 grit give a smooth finish ready to he sealed with epoxy, and finish coat of whatever you like,