Wildwind,

The boats on our lake use auger posts with cross members (just like an "H" section of dock hardware but with an extended center), cover the cross members with PVC. Pull the 16 up on the PVC covered cross members, don't forget to tie it down.
The good thing about the auger posts is that if you add 16's all you have to do is connect two additional cross members to the original rack add two more posts and you are there.
I currently have two on racks with this system. Works and looks great.

John