I would just use bar-end levers. They're cheap and light, and feel very comfortable on a bullhorn bar. Basically it just plugs into the end of the handlebar and then expands for a tight fit.

I know you want the lever a little further down, but I don't think it's really a position where you'll have your hands when you want to brake. Plus with the bar end levers the brake hangs down far enough that you can reach it easily from about 3" from the end of the bar.

Have a look at the photo from Performance:
http://www.performancebike.com/shop/...tegory_ID=5225