This is why you don't use a cliff as a part of your wall. The planning is just... painful. The other reason is that there's a bug exploit that keeps popping up that allows players to jump over cliffs.
The answer is here:
viewtopic.php?f=2&t=16013&p=251953#p183673 (second post from the top)
Just in case you're not sure what he means:
What you CAN do is place another starter post in there. Hold down shift while placing the sign, and you can move without the grid alignment. Just put a single item into it and don't build it. Test the closure by walking around the post and doing everything you can to squeeze through. If you can walk through, so can anyone else. You need to remove materials and try again. Once you're satisfied with the positioning, you can finishing loading materials and build it.
Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.