I intended to fit flush hatches as I think anything else spoils the lines of the kayak.  I decided on a forward hatch 16" long, tapering from a width of 12" aft to 8" at the front, with rounded corners.  The stern hatch is a sort of hexagonal, 16" long and 14" at its widest point.

I cut out both hatches with the jigsaw.  This was not at all difficult, but I was very careful.  I added strips of 2" glass tape to the inside edge of each.  I wrapped both hatch covers in cling film and taped them in place on the deck.  I mixed up some epoxy with added toner to make it black, and applied six strips of 3" wide cloth along each inside edge.   This took about one hour per cover.  The inside of the deck looks a mess because of the black colour, but then, nobody will see it !!

My main worry at this point was removing the covers.  I wasn't sure if the cling film would allow for the easy release of the cover and the book does caution on not leaving the epoxy to cure too long.  As it turned out, I needn't have worried.  I left the hatches to set for five hours ( the outside temperature was around 150 ) and I inserted a small wedge between the cover and the new flange.  Much to my relief, the cover fell off immediately.  Now the covers fit perfectly, with a narrow black edge all around.

If you decide to use black epoxy, be aware that you will end up with a black line all around between the edge of the hatch cover and the deck.  This is thin but very visible, especially if the deck is light coloured wood.  Also, any variance in the width of the gap will be more evident.

Lessons learnt

My usage of the cling film was not perfect.  It had many small creases after I applied it and the epoxy mimics the surface very well, so there are many small creases in the flange !  These are not big enough to cause a problem with sealing, but I should have been more careful applying the cling film.

I had rounded the edges of the hatch covers.  Fortunately, the rounding was only slight, for I would have had difficulty removing the covers had I mad them half-round as the epoxy would have seeped around the curve.

I did not allow for the thickness of the seal when taping the covers in place before glassing. This means that the finished covers sit about 3/16" proud of the deck. Next time I will tape the covers 3/16" below the level of the deck so when the seal is put on the covers will be exactly flush.