This is a good thread that goes into detail on best practices.
The thing you may need to adjust based on your description are your boxes. Try to think of them as digital trip-wires, creating long thin boxes which allows the target size to remain high with low sensitivity.
Outdoor cameras will always be subject to more false recording hazards in general. You’ll never filter out all unneeded recordings with VMD unfortunately, but you can stop a large percentage usually.
If you are still having trouble after reading over that thread, send us a private message with an example screenshot of the camera view and the VMD boxes. We can give advice specific to that.