Warthog - This is how chuck cored the bottom of his bracket.
I used a bunch of 6x6 inch squares and cored the center on mine. For the outer two visable/exposed sections of the platform . . . I made these two molded parts to "stick-on" like stamps . . . that way the bottom of the platform will have a nice finished look to it.
You're making me think twice about my choice of hatch, but I've used one of these Bomar hatches on my brothers boat in the splashwell. It has been water tight since 2002 when I installed it and he keeps his boat on a mooring all summer (it's got a LOW transom too). The bracket should be very high out of the water . . . and with 5200 around the lip it shouldn't leak.
This was how they cored the hatches on the boats originally. only they did a better job.