I can see all the photos.
I see a fairly low upstand around the openings, but no lower than is found in all the aluminium forehatches for sale. These have an upstand that presses into a soft rubber seal in the underside of the hatch, which is compressed when the hatch is dogged down, so this is what I would try to recreate in Somerled's cockpit. I like the general design concept, with the steelwork sloping so that it sheds water and the hatch having wedges to support horizontal slats for sitting on. I think I would do the same again, with the addition of the rubber seals. To close them, I'd use some over-centre catches like this.
I don't think the cockpit is excessively vulnerable to being filled by a wave, but I don't see any large drains in case it is. I'd like to put in two very large tubes from the cockpit out through the transom (4in dia wouldn't be too big). The bridgedeck is high, so no vulnerability there. A deep cockpit feels safe at sea, so I wouldn't want to fill it in, and it doesn't look as if you need extra stowage space with boxes within it. I think it will be seaworthy, if it can be made to drain fast enough.