Little space to hold my workpiece on

At present I'm coming to the end of my latest build of a 'Mini Mill' and was trying to figure out how to hold my workpiece to the gantry plate. As its only 5 X 5ins square there isnt a lot to play with. I was intending to cut coin reliefs (not UK stirling) but things like plaques and small figures that would fit on felt covered boards and frame them. Then people could mount it on the wall.

Like a presentation plaque of say 'Captain America in the middle with a discription of who the guy is and a couple of his buddies in the films he's been in, Spider man, Irion Man, The Hulk (I'm aware there all under copyright and its just a suggestion of what you could do in other areas).

But how do people keep the workpiece in place without it slipping around while being drilled.

Thanks.