The smallest, cheapest and simplest waterproofing is a ziplock bag. For sealing up any cracks, check out "self bonding silicone tape". I saw one person use a clear plastic drink cup with lid. You can get a genuine outdoor camera enclosure with front glass for under $15: http://www.monoprice.com/Product/?p_id=6687
There are a number of other outdoor camera enclosure examples on this thread: http://www.raspberrypi.org/phpBB3/viewt ... 43&t=51187
I think the usual strategy is not to make an enclosure completely waterproof unless you are using it underwater. Humidity can still seep along inside cables and condense inside, given enough time. Just leave a drain hole at the lowest point and it should stay dry enough.