This may give you an idea.
http://www.billinforres.110mb.com/Obsy/Obsy1.htm
However the smallest footprint I have seen is just to put in a pier with the mount on top. Then to take a normal sized plastic rubbish bin. Invert it over the mount for protection and then mount the lid so that it is around the pier on the bottom of the inverted bin. The metal clasps that seal the lid to the bottom of the bin will then hold the bin over the mount and pier giving weather protection. If I can find the site that demonstrates this I will pm it to you.
Have a look here for more ideas
http://obs.nineplanets.org/obs/obslist.html
Allan