My topload is 4.5" dryer duct formed into a circle with an outside diameter of 22.5". I covered the duct with MANY layers of masking tape to smooth out the ridges.

The masking tape was then covered with aluminum tape strips. The end result was a very smooth toroid, designed to promote small numbers of large streamers.

A piece of thin plywood cut to fit the center of the toroid completed the assembly. That was covered with aluminum tape and then the whole topload was smoothed one last time to get rid of any obvious points or projections.