I cut a piece just that size recently. I screwed it down with wood screws into a larger piece of sacrificial MDF, then clamped the larger MDF using normal clamping. If you had a couple pieces of MDF the same size, and a way to set them into the same place every time (dowel pins, etc.) you could swap that way and have the next ready to go. I usually make 1/4" dowel pin holes with the SO3, I drill the holes into MDF, then set the first piece just where I want it, run the same NC program to drill holes in that (and every subsequent piece) in the same place, then place 1/4" steel dowel pins in and always have the same position, works good for 2 sided jobs too if you place the holes along a center axis. Or as Adam pointed out above, you could cut a pocket the same size as your stock and use tape.