I agree with kchriste: just use a pole, maybe with a bright red flag or something on top. No power worries, no RF worries, no special enclosure requirements, etc. Oh yeah. No unscrupulous hobbyists stealing your nifty little devices for parts.
In Finland they have a good, cheap method of locating the sides of the road for the snowplows when the snow is too deep to see what's what: they just put sticks in the snowbanks so the plow operators see them poking up. Problem solved.