Because milling & drilling PC boards does not use a lot of lateral force, light clamping ia sll that's required.
Are you doing a production run of the same boards? If yes, then make a fixture by cutting a pocket the exact size as the boards, and as deep as they are thick. then a thin rectangular frame that is larger than the boards by 1/4" all around, and with an inside the same shape as the boards but 1/32 smaller!
Then drill holes for a few small screws to use the frame to clamp the entire board in the pocket! all low profile!
If you have a lot of boards, make two or more pockets with clamping frames!
Now every board will go to the exact same place for a fresh start of the program!
Save the fixture for future runs of that board!
Hope this helps!