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Most people who make homemade snake repellent, use mothballs (naphthalene). For what it's worth, the government says there is no known snake rellent that works. (however, the EPA has approved "snake-a-way', the only approved commercial product to get rid of snakes; it contains naphthalene as the active ingredient)
I will give you a couple of links that may help. Bear in mind, that the only proof that homemade repellent works is anecdotal evidence.
http://www.dnr.state.mn.us/livingwith_wildlife/snakes/removing.html -DNR site
http://www.pestproducts.com/snakeinfo.htm -Snake-A-Way info, also sold at Home Depot
http://www.blufftontoday.com/node/7553 -discussion forum thread on this topic