Aradia's ballgown design so so pretty. Good luck with it!
I am honestly not sure how to go about making this kind of skirt, but I went into depth about some ideas (and linked a few threads and tumblr posts) in this thread:
I also found a Terezi who explains her process on Tumblr:
While it is a somewhat different look than you want (as this hoop is enclosed by fabric on the finished dress), there is a Roxy who talks about her method on Tumblr, as well:
I think that you would be going very much in the right direction if you used ivori's method with some added vertical support (maybe put something stiffer in between layers of drywall shims?)
I hope all this helps!!