It should not be too difficult with a couple of pieces of ply cut to shape (take off a bit extra to allow for the matt to be added later.
Cut hole for hatch (again cut the hole a bit bigger).
Join ply together, you could simply use cable ties following Narcosis' method in his searider thread.
Glass over all areas of the ply with resin and matt, three layers or so should suffice I would have thought. Cut out cable ties if you used them and fill holes
Depending on the finish you are after, sand and fill, sand and fill etc till smoothness reaches desired levels.
Glass into boat and then paint or flowcoat (or, paint or flowcoat, then glass in and paint or flowcoat over the new glass).
Get some MDF, cut to shape and join, wax inside, gelcoat, then put down matt and resin to required thickness. Once dried, release, glass into boat, flowcoat new glass if necessary.
Would expect that the first method is easiest.
Search on consoles and you'll find lots of advice on how to make those, similar methods.