There are a number of creative ways woodworkers fasten workpieces to a CNC spoilboard. Some create 90o fixtures that are permanently mounted to the spoilboard to orient the workpiece and use cam clamps to push the workpiece tight up against the fixture. Some use aluminum t-slot rails spaced between strips of MDF. Some use double-sided carpet tape to hold the piece in place.
The E3 CNC Router uses fifteen ¼”-20 inserts with ¼” threaded bolts and wing nuts along with 8 clamps.
The E4 spoilboard bed has six T-slots. The kit comes with 1/4"-20 screws, t-nuts, and wing nuts along with 8 clamps for mounting the workpiece.
Whatever spoilboard configuration you chose you need to be sure the clamps and fixtures don’t get in the way of the spindle while it is cutting. Also, make certain your Z-safe height is high enough to clear the hold-downs when the spindle is making rapid movements above the workpiece.