A substitute in the meantime might be one of these:
https://www.amazon.co.uk/Silverline-...ds=swivel+vice
I recently bought my second one from Aldi, which is identical to this Amazon one but cost only £6. It will enable you to get work horizontal.
If it was me I'd mount the ear-ring for setting on a T-shaped plate (one you've made yourself for peanuts or the overpriced alu one that typically comes with the Benchmate system:https://www.hswalsh.com/product/grs-...chmate-004-106). Drill a small hole through the top plate to one side of the T's vertical (or, better still, down through the vertical), gloop some hot polymorph on to the plate and sink the earring post into the polymorph, passing the post through the small hole as you do so. You'll be ready to go in a couple of minutes.