We have the problem too. To date some owners have had metal structures about six feet high and equipped with tempered glass or Lexan. We put up a net. Golf netting is available from a number of sources. The netting works best because it allows air through but not golf balls. You can see through. It is light in weight so the support structure does not have to be heavy. It can be any height the condo association will allow. For our patio the preferred height is ten feet. Total cost for materials to construct a ten feet high by twelve feet wide net and frame structure is under $200.00.