Here are several DIY sites I've bookmarked

http://www.familyzoo.us/how_to_build_an_incubator.htm

http://www.herpo.com/cages/incubator.html

http://www.arbreptiles.com/cages/incubator.shtml

http://www.duf.net/reptile/Docs/incu%20design.htm

http://www.eyehit.com/degei/supportf.../inc/index.htm