Roof vent mounting ideas?

I'm at the point where I'm framing for the roof vent, and I'm a little stumped. I've got a 14X14" vent, and if I frame up even and perpendicular to the ribs, then the roof is going to be low in the middle because the radius of the sides makes them higer by about 1/2". If I radius the supports, then the vent will rock on top of the mount! There's got to be some happy medium that won't leak.
Everyone did so well on the mattress question I posted a few weeks back that I thought I'd try again.
Thanks!
jb
Everyone did so well on the mattress question I posted a few weeks back that I thought I'd try again.
Thanks!

jb