A local metal fabricator gave me a couple of worn out band saw blades. I cut each in half and form four 21" diameter circular dams I use to fuse sink blanks. I line the circles with fiber paper and use thin nichrome wire to hold the rings together, allowing a little extra space between the glass and the dams. The nichrome wire locks into the serrated edge of the band saw blade helps to keep the dam from slipping open so I don't need the tabs.have the form made as an untapered cylinder, but split it vertically, and have tabs welded to the outside of the split that would let you wire it tight. After the pot melt, you would cut the wire, and gently open the ring. It pays to make friends with good stainless fabricators!
I also trim the flat rim off the bowls so the glitch created by the overlapping blade and fiber paper are not a problem. If the rim is to be kept, the glitch can be ground clean,