You're making me nervous now (not for myself, but someone who asked me if their setup is good). 2 house walls and a 3rd that's deck railing/fencing, where the 4th wall is tarps that are unlikely tight enough to be kosher - is there a tzad the deck/railing wall forces you to put lavud strings on the 4th wall? And if there's a tzad is does, is there whom to rely on not to?