Yep the horizontally stacked yagis are a contender for sure. I seem to remember there was a rule of thumb for the minimum separation when you stack yagis - some percentage of the boom length I think. I wonder if you could horizontally stack 3 of these yagis in a 20m space? If we assume 1/8 wave radials, that leaves 15m to stack 3 yagis, or a separation of 7.5 meters between yagis. I wonder if that is too close?