I've made my own 8x10 from foamcore and rubber bands ...
http://www.flickr.com/photos/dairequinlan/4300741225/

The bigger you go the less precision needed. Though this is completely lacking in any movements. When I ever the chance to revisit it I'll do something a little more sophisticated.
A couple of the better shots I've gotten out of it:

http://www.flickr.com/photos/dairequinlan/4386036602/
http://www.flickr.com/photos/dairequinlan/4386033060/