Get some 20 or 25mm Polyethylene sheeting from Maizeys Plastics and cut them out with a jigsaw. The stuff machines very easily. You can use a router to round over the edges.
You can also drill a hole through and insert a stainless bold and nut. This will allow water to drain freely, if you seal it up completely there is always a chance that water is trapped and your but always wet.
