Rivonia Village & Square comes to mind, there's 3 parking lots in that area. Also worth trying Thrupps centre in Sandton, and even like Mondays or Tuesdays at Monte, sometimes you have the whole side of a parking lot to yourself, just ask the guards if they can move the the 'park here' signs and go up a floor or two. Randburg Square always have issues with their boom gate there, you can go up to the roof and then down into the underground parkings from there.