Look at the 4th or 5th on down...16.99 brand new!! I had one that worked amazing, perfect for what you need..and fits nicely onto the BB shell. Pretty light too.
I've never had a problem dropping the chain on one up front bikes. Though I normally run the chain a little shorter than normal to maintain the tension a bit more.
I think I'll just pay the extra few quid and strap an actual mech on. At least that way it's there ready for when I finally decide to run a twin or triple up front...