Midget,

You definitely want to secure the motor pylon to the deck with a fastener. I personally would use blind nuts instead of screws. Screws will loosen up also under vibration over a period of time. And if you over tighten them, they will come out even sooner.

Use blind nuts with a thread locker on them. This will keep them from coming loose on you. The last thing you want is your pylon assembly coming off the main deck.

I would also put a small piece of thin ply under the coroplast also. This will make it harder to pull through the coroplast. In other words, sandwich the coroplast between the ply's.