I really don't understand your question? The big cog in the back? Why would you be getting any slippage there?
Ok I reread your post, you are talking about the single chain ring right? Cogs are in back chain rings are up front. There are a few options.
Expensive:
go here and get a chain guard:
http://cyclocrossworld.stores.yahoo.net/chainrings.html
Cheap:
Grind the teeth off of an old worn out ring to use as a chain guard. Either do two rings or one for the outside and get a dog fang for the inside
Middle of the road:
If you already have a front dérailleur then just use that and use the set screws to adjust it so there is no rub on the small and the big cog. Drop it down really low.