you need invisible blockers..
read my elevator tutorial to see how to turn view blockers on and off..
then you need to use wall 620 and 621.. they look blank, but are invisible wall blockers that cannot and can be shot through ( for things like chain link fencing etc.. ) respectively..
there are other types of blockers for scenery, scrolling, and so on.. open one of the existing maps and turn on view blockers and have a look at them..
if you select one, and press p, it will find it on the selector bar.. then make a note of the number for future use..
different types are in different categories, like scenery, walls and misc., so if you can't select a blocker, try a different selector category..