What is the best safe way to close a hole measuring 2 cm in the heart between the two ventricles?

This is done surgically, and usually does not require open thoracotomy. But you should discuss your particulars with your cardiologist directly.

Surgical only. All details matter and timing and actual procedure are to be discussed with cardiology and cardiovascular surgery, long term goals as well as benefits and risks of having procedure done vs not having it done are important items to be reviewed in your medical context.