I had the opportunity to participate in Camp ShoBot at Techshop a couple of weeks ago and learned about a great way to hold material on a ShopBot.
Not only does it allow you securely hold your material, but it also allows for 100% dead-on repeatable registration for your part.
This only takes about 20 minutes to machine.
Step 1: Draw the files in vCarve or Aspire
Using 3/4" MDF, you are going to cut a pocket that is 1/2" deep.
Place the thru-holes for screws and pegs on 1" centers. Make the holes for the pegs 3/8" in diameter. This will allow you to take this jig to any machine and register it. All you have to do is to place 3/8" holes in the same spot as the jig holes and use a dowel rod.
You also want to draw a square that is the size of the material that you want to hold. This jig is designed to hold a 3x3 block.
The trick to this is is to make the left and bottom sides of the pocket that you will cut parallel to the X & Y axis. The top and the right edges of the pocket are +7 degrees from horizontal or vertical. You want these vectors to be 1" greater that the top and right of the square that represents your material.
The next thing to do is to model the wedges that will slide between the sides of the pocket and the material.
You want the angle on tho wedges to be 87 degrees on on side and 90 on the other.
I've included the files so that you can cut them out.