Ingenious but if expensive perhaps you’d rather think carefully if you can’t hold the occasional awkward work-piece with what you already have, aided by a little lateral thinking and perhaps a bit of quick “scrap-box” jig-making.
It would take a lot of cleaning afterwards if the milling has produced a sizeable heap of fine-grained swarf!
A better way to hold a small attachment like that on a milling-machine would be a base-plate extending beyond the tool itself, with holes for T-slot studs.