Chuck writes:

The span between the posts will be 20
feet. I'm thinking that some 2x4x.120 rectangle tube should be strong
enough for the horizontal, but what I need to know is will the tube
sag under it's own weight

My screen enclosure (Florida, no snow, but hurricanes) spans about 24' with
2 x 6 aluminum rectangle tube. I expect it would hold my weight, but I
sure wouldn't want to try it.