Transom on your sailboat is designed to hold the hull out at the designed shape. It also stops water from entering the boat when the following wave crests and breaks on the transom. Waves have surprising force that a proper transom must hold back or in the case of an open transom designed boat the wave enters and then drains back out.the transom being plenty strong, even with a big hole cut out of it.
With the hole you have created a place of entry and no exit. If you just repair with a "thin skin" of glass and resin you put the boat at risk of inundation by an ill timed wave. Of course if you never take the boat out of the bay or only sail in gentle breezes, you need not worry.