I used 3/4" MDF for my spoilboard.
I used threaded inserts that are ~1/5". I used the CNC to cut the holes for the inserts and then screwed the inserts into the board so that they sit inset to the top of the board.
This gives me about 1/4" of clearance if I overcut my part.
Using the threaded inserts kept me from having to take the spoilboard off the machine to install the t-nuts from the bottom.
You can read more about mine here: