Something I've been doing each year !
  I made single piece hatchboards of inexpensive plywood both front and rear.  I then painted them with house paint.  If you do this as one piece, it is a bit heavy, but easy to deal with.  You could make it in two or three pieces.

  I leave these in while the boat is put away for the winter, either in the water, or up on the trailer.

Saves the varnish/cetol !