Here I am clamping by screwing into the wood. I drilled a pilot hole for the wood screw with a hand-held electric drill. I tightened the wood screw until I could hear the wood cracking.
But still I was not clamped tightly enough. In the picture you can see that the tool path has veered to the left as the Delrin has been forced to the right. Fortunately the part is not scrap.
I will re-cut the wood area with an island for the rectangular hole to sit in and try again.