I would imagine, small screws into a piece of wood that can be held in your vice.
The screws would be smaller than the ones to be used, so any engraving could be done tight to the countersink.
This way, it could be removed, and replaced without issue.
At least that is what I have done on holding small pieces.
I have tried everything. Epoxy to a T plate, holding with the jaws itself, using layers of protective material.
The wood backing with small screws works best where it can be used. The engraving process itself does not jar the work that much, but, it does need to be stationary.
I have tried pitch. The denatured alcohol and resulting polishing minimized my graved surfaces. I have tried shot pads with some success. I keep moving with the piece, but it can be set directly back. I once made a table with an engraving vise that I could rotate with my foot. I could change angle, and rotate it. However, my Victor ball, with no stop has less resistance to turning than anything else, yet, it does work great.
You gotta figuire out what is needed for each piece. Pieces where you continue the engraving from the side pieces to the inner patch lid, you gotta fix it so you can continue the lines.
I am still working out bugs on all of this.
Sometimes I even make something I like.
Sometimes.