The guys on the forum have already provided you with some good ideas on this but, I thought I'd add one more. It is in the same thought process. The hatch described was made for a 1M Seawind RC Sailboat. There are at least two of these being made professionally and sold here in the states for this boat, as it is famous for having a leaky hatch. This idea can be adapted for your applicaton and would work for your boat as well. This was made at home for a lot less.
The hatch in the picture is made from clear acrylic 3/32", some nylon "thumb turns", nylon nuts and washers and epdm door gasket material, all from Home Depot. Cut the cover to fit, carefully, (plastic can be brittle) drilled 4 holes in the hatch, installed the epdm gasket material on the boat deck (it is adhesive backed), marked on the gasket material where the holes in the hatch were, melted holes in the gasket with a soldering iron, drilled the holes in the deck where the melted holes were, superglued the nylon nuts under the deck, painted the thumb turns black to sort of look like winches and then bolted the thing down. It is totally waterproof, easy to put on and off, and clear so you can see inside the boat to see if everything is OK. In addition I suggest mounting an exterior switch water proof switch, so once the batterys are in, You won't have to open up again until the day is over. The hatch cost less than 20$ into the whole thing and that gave me enough to do two hatches! Tip: add some vaseline to the nuts threads before gluing to prevent the super glue from messing up the threads.
If you draw the hatch lay out (draw on the plastic) on a sheet of plastic, use two of the factory edges for 2 your edges, and you eliminate a couple cuts. This one was cut with a small saw like a hack saw and then use a dremel tool to finish off the edges. To get a nicer finish you can use a hand sanding block with varying grit of sandpapers, then finish up to 2000. It does almost polish it but it certainly gives it a finished enough edge for me. Another hint is to put vaseline on your threads of your “thumbturns” when you superglue the nuts to the underside of the deck. That way it any superglue does get on the threads it wont stick.
I must give credit Mr. John Ebell, a contributor on the RCGroups.com the credit for this information. He and I were sharing information on a new version of this idea that was seen on E-bay. This is his fix for the problem. I attached a picture of the factory hatch, which looks like it won't leak, but it does.
Good luck
Windchaser