The last two fixtures got fused together because the metal filled the cavity where pipe diameter didn't quite match the flask. I managed to beat the chunk of metal out of one of the fixtures, but both got destroyed in the process.

Just when I was in talks with a plant that manufactures custom riser/filter combinations from CAD drawings, one of the engineers here at the hackerspace got interested in the problem and helped create a fixture that worked to increase casting quality:

The new fixture uses a ceramic blanket and more precisely machined & welded pieces (now I know how to weld!)