Rather than a shirt, I would suggest an outer garment. You can get a loose mohair sweater in a large size that will go over anything, and if you wear nothing underneath the sensation can be very special indeed. No problem for hands to reach up inside either, from the front, or back around the body.

There is a garment that is worn on boats, like a plain cotton windproof shirt that doesn't open all the way down and has a high hoodless neck without a collar. Similar garments can be quite fashionable these days, and they are quite loose and comfortable.