How to make a big boat hatch?

hi there, i need to get hold of, or make a large boat hatch big enough for my 175 horse inboard to fit through. I have extended the cabin, and now cannot get the engine in! Live in u.k., cant seem to get a big enough hatch ( would be too expensive anyway) so thought maybe i could build one if anyone had some ideas? Is for a grp boat to fit in the middle of the cabin, needs to be roughly 3ft by 5ft, am not too good at wood work, but could try, perhaps perspex in a wooden frame? Any help appreciated, thanks

 

A picture of the problem might help?