I had a Nacra 6.0 NA and had square tops for years.
The first was made by Calvert sails in Florida (Pentex Optic)
The second was made by Elliot Pattison (Dacron 6 oz)
y preference was the Dacron sail as I was not racing and it could handle the rolling of the sail and flogging while on the beach. I also liked that the had a bottom batten at the foot and the Calvert sails did not which caused excessive flogging when I dropped the mainsheet blocks and let it sit on the beach between sails.
Never had a mast problem with either set of sails