This fits into the slot in the base of a Dremel Workstation (220-01).
This might help stabilize a work piece.
I upload this in the hopes it will give others ideas for similar accessories.
Possible future improvements in future parts (room for others' innovation): a taper to make intital fit easier; a way to tighten it for extra stability; a part that grips while swiveling.
The file format is brl-cad.
This has very tight tolerances and not all dremel bases have exactly the same dimensions, so you may need to file it a bit to make it fit.