Well, basically in hl the easyest way is sprites, or transparent textures,
the testure way is easyer:
Get a textured brush with a bush texture on it or something,
turn it into a func_illusionary (or func_wall) FX ammount to 255, Render mode to 'Solid'
Now copy the entity and make a cross with it in top view:
_|_
-|-
something like that