you have to go through all the objects, splats, splines and tiles to find what you want.
eventually you'll get familiar enough to roughly locate what you want from memory.
what you want should be in ambient objects > walls > concrete or metal
I don't know if you managed to find the metal fence you are looking for, but it is in the walls/stone section. It is actually a concrete fence and it is wire. Hope that helps. I can be more specific if you want, but I think you will find it fairly easy.