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Knee braces for the corners.
I'm in the middle of building a treehouse with this as the base for my blueprint. Here is the key piece for the brace. I put one 2x6 on the slot with two more on either side for added structural support. I'm not a doctor but so far so good. https://www.menards.com/main/tools-hardware/fasteners-fastener-accessories/structural-connectors-reinforcements/structural-hold-downs/hold-downs/usp-structural-connectors-2-1-2-x-9-3-8-holdown/p-1444445531014-c-8852.htm?tid=-6496288680977047586
More pictures to follow. Such an awesome treehouse plan.