Since you're going offshore with the opportunity to experience some challenging conditions, I have three or four suggestions for your consideration.
When you build these boards, they should not wobble or rattle in the hatch frame's channel. So.... if you angle cut, rather than notch cut, the boards, the top could possibly pinch itself into the lower board. A proper thickness should prevent this happening so the boards have very little play. If you go with the angle cut, making it shallow, maybe 10 degrees, but remember the bulkhead sits at an angle, that should be added into the cut angle and you'll end up with a deeper angle than you expect... another reason to opt for the notch cut, which is easily made with a few passes on your table saw.
For a secure offshore set up, it should be possible to lock each board in position from both sides. Not at the same time, but so you can lock them in place from either inside or out. Simple sliding latches, a long rod, etc. there are a couple of designs you can use. If the boards are locked in place, a knockdown will present less risk of flooding... if they are attached with a snap on cable or pendant they won't disappear if you capsize... I know this sounds alarming, but it isn't going to be that much more work to build this feature into the boards.
This thought has reminded me of the importance of the "bridge deck" designed into most offshore vessels. A bridge deck protects the water from sloshing into the cabin should the boat get pooped.... so...... you might consider making a gasket or seal for the lower board. Oh...... that's another reason to go with three boards instead of two. The boards on my little Catalina 27 are cut so the bottom of the three is exactly the height of the cockpit benches. This allows you to leave the bottom board in place and still get into the cabin or maintain good ventilation in hot weather. Since you want this board to act as a "bridge deck board" it will fit somewhat tighter, it's just easier to leave it in place should the conditions warrant.
Final thought, this is related to the previous one, where we're concerned about getting pooped or a rogue wave slapping you at night and filling the cockpit with water. I suggest you examine your cockpit drains..... at the very least, make sure they are clear and flushed. If you have those little grates over the drain holes you might consider removing them....or at least be aware how easily they can get blocked up. If you are really into it you might even want to install another set of drains.....or enlarge the existing ones... the idea is to evacuate the water in the cockpit as fast as possible.
Years ago, I read this book about small boats that had made circumnavigations. There was a guy that did it in a Cat27 ( I can't imagine an intelligent person doing that, btw) and there was a list of upgrades he made to make the boat somewhat safer for going off shore. At the top of the list was enlarging the cockpit scuppers....because Catalina's, in general, have large cockpits....'cause they're coastal ya know and people like to entertain........... I always remembered that point... so easy to do... yet.... why do the designers not consider it??????
Okay, have fun.
