To hold the pieces I would cut out the shape of the pieces, in 1/8" ply so it just barely fits each piece. I would include a small notch in one side of each cut out to allow easy remove of the items after etching. Then glue the cut out ply to an uncut piece of ply to make a floor (optional step). How you register your jig in the bed is going to depend on what kind of bed you are using. I have an origin jig in my laser where I took a small piece of ply and push it all the way into the top left corner. Then cut the ply from the origin in the x and y directions. This allows me to place the corner of my work piece at the origin each time. Here’s a pic of my origin jig.
