If you're OK with cables swaying in the wind, a flip & chop is an inexpensive solution.
While doable, drop bar levers tend not to do that well on bullhorn bars, particularly if you run the cables under the bar tape. There's a sharp bend they don't see otherwise, and the available stroke before the levers bottom out against the bar is less than on a drop bar.
But if you turn your bar upside down, you get to choose how far up to slide the levers to get required stroke, and a sharper bend than bullhorns/base bars tend to use. You can test ride a few different positions to see if they work out for you. Then you cut the bars above the levers, apply new bar tape and you're done.