At first it sounds like a trophy that would take a long time, but in reality, it only takes about 10-ish minutes.

The easiest way to do it, is by going into sandbox, pick the Ventelieu map (Don't touch any of the settings).

After that, you pick a fence and start spamming down. It's faster if you move your camera as you place so you can please faster. Even when you it 0$ at the right-bottom corner, you can still place fence down, which means you will be able to place fences forever.

After that it's only a matter of time before you've placed enough fences and get the trophy.