I think with about 20 fences and 3-4 pit traps you should be fine. I completed it yesterday, first I made a 10-fence with 1 pit trap and it didn't work, then I made a longer one with 4 pit traps. For it to complete, you need to zoom out: so after you put 3 pit traps in your existing fence, you can go iteratively by making a fence, zooming out, making a fence, zooming out, until it works.