The area you are working with is large enough to get a good hold down with vacuum. You would need a good vacuum. If you are putting a lot of holes through this may not be feasible but if it is mostly surface features and few holes you may be able to isolate the locations with O-rings in the vacuum platen. Ideally some pins around the edges would be good to stop the piece sliding sideways under the cutting force.
Alternatively if you are doing many holes but not much in the way of surface features you could try use a 1/2" Al top plate over the whole part. You simply machine clearance holes where needed to do the machining on the work. Of course if there are too many holes this doesn't work.