I think that the easiest and cheapest way is to use bar end shifters. They come with a friction shifting option so you don't have to worry about compatability issues.

What they don't normally come with is down tube cable stops which you will need. Fortunately, every set of aftermarket STI levers comes with a set of cable stops. Consequently, most bike shops have a drawer full of them. If they like you, you might be able to pick them up gratis.