I could be completely be misunderstand what your trying to do, so that my suggestion with a grain of salt.
Basically, piers to me is a dock on the water.
I'm think piers as in footers, so if I'm correct this might work for you.
You could just make hex or octagonal forms with 1"x4"s or the like then use them with the sonotubes. Just sink the tubs and use the form on top to get the shape you want that's exposed to view.