There's this stuff that jewelers use for holding onto irregular parts, it comes as loose beads of plastic that you put in water and melt in the microwave or over the stove, then form around your part. It can be reused indefinitely, as long as you don't contaminate it. Form it into a square on the outside, and you can hold it in your vise or 4-jaw chuck. https://www.riogrande.com/product/je...ompound/118221