The panel behind it is malemine (spelling?) you dont need that though you can use MDF under it. I would put the hatch you have down on a piece of MDF and scribe twice on two peices. cut the inner section out of both of them so you have 2 outer templates. screw, nail or staple them together one on top of the other. get some sculpy putty and raduis the hard edge on the bottom like the other hatch. wax the mold. get a pint of gelcoat from any marine store. roll the gelcoat down mixed with MEKP. then 2 layers of mat for what is called the skin coat, it is important that you get all the air out of these coats. then once that kicks, 2 layers of 1708 biaxial. and depending on wether or not the hacth is cored then you would core it with some 3/4 ply would in the middle. ( I have some marine play in 4"x4" squares that you are welcome to. and a couple mor layers of 1708 and you are done. here are some hatches I just made so you get an idea.
http://www.classicseacraft.com/forum...=&fpart=4&vc=1
half way down the page you will see the molds on the table and the layup.
put some detailed pics of the hatch you still have and maybe some other can chime in on how to make a replica.