I’ve been using an idea i read here somewhere.
I predrill on empty spaces as a first move in the cam.
In the modeling i make a 20mm donut (2x6mm so the bit can pas on both sides, and 6mm for the head of the screw, and 2mm extra to be safe). I copy it a few times across the workpiece, if it fits in the empty space, it is safe to screw there.
Then in the cam i drill a 2mm deep hole in the center of the workpiece, with a different tool, so the program halts afterwards. Then i screw the skirts in place, place the board, let it drill the holes, screw down once the sled clears the hole, et voila, workpiece held down, and very little chance of my bit hitting it.