All you need is a vent with a top, make sure the edges of the vent are raised 1/4" or so above roof level. Then secure a top that will overlap the edges of the vent a couple inches....you could always install a fake air register and pull air from the bottom of the attic through it also....you could also just use a vent that looks like an upside down J with the intake pointing toward the roof top.

Many options....don't forget to seal it up good.

Unless you have a large operation then don't worry about choppers, if that's too much to deal with for you then consider another hobby.