our local hardware store carries these thing for putting them through the roof, it has a funnel shaped hole in the middle, they work great for the sides too. they help keep water from melting snow on your pipe from running into the shack.
they look something like this, and i think they are about 15 bucks.
just make sure your siding goes over the upper edge so it doesnt leak water there either. if your shack is already sided, youll just have to slide it up under like a 1/4 to a 1/2 and inch or so.
i hope this helps.