Not sure if this will work or not, but I remember reading about this in Tape Op some time ago. Technically these are baffles, but it might be worth a try. Go buy two solid core doors and mount them to a rectangular piece of plywood with brackets so the doors are standing straight up. Wrap some egg carton foam around the doors and secure with construction glue or some other type of caulkable glue that will stick forever.
Like I said, these are baffles but they might stop enough sound to prevent your neighbor from hearing. Good luck!