I had a Ritchey Logic Comp adjustable stem. You'd have to pretty much disassemble it to change the angle, but once it's put back together, it's not going to slip at all. If 45 degrees is enough angle, and 120 mm is long enough, it's the way to go.
http://www.ritcheylogic.com/dyn_prodfamily.php?k=98293
Wide shot of it on the bike: