Mike, Etienne has a great website for doing the plywood coaming.
http://etiennemuller.com/northstar/index.html

If you go with the plywood coaming/cockpit, you'll differently need to either widen the cockpit recess or narrow the cockpit coaming. The plywood coaming has a wider lay up for the riser section vs the standard woodstrip riser of 1/4 inch. Maybe 3/4 inch wide or more, so this needs to be consider on both sides.

So if your cockpit recess is 19.75 inches wide, you need to substract at least 1.5 inches for the width of the plywood riser; therefore, you need to have your cockpit width opening 18.25 inches or less.

Good Luck K.Greer