If I'm understanding your question correctly, you are asking how to make a circular form? While I have not tried the boiling glass yet, I have thought about it and have made a circular form like you're referring to. With the high heat (1700F) I wanted to stay away from SS as it has a tendency to spall.
I have made fiberboard molds already to do this. You can make whatever sized circle you require with FB. Depending on the type of fiberboard you're using, you are going to want to kiln wash the molds as some FB have a tendency to create a lot of dust. You may also want to think about using a ridgidizer before applying the kiln wash.
Fire the form by itself before doing your project.
You can then line the interior of your circle with 1/8" fiber paper, cut your glass and go from there.