At some point in the past, a link was posted here for (relatively) cheap clear plastic domes to be used as a robot head. These are actually bird feeder covers, now sold at birdfeeding.com:
http://216.247.247.169/miva/merchant.mv?Screen=CTGY&Store_Code=B&Category_Code=Bird+Feeding+Accessories At $35 for an 18" dome, it's about 1/2 the price I've seen elswhere for things like camera domes. These will have a hole at the top, while the camera domes have a lip around the rim to fasten it down.
I was wondering if anyone had actually bought one of these and could give feedback as to how it worked. One thing I'm interested in is how clear the plastic is. The description says it is UV stabilized, and it looks a bit tinted in the picture.