Find a large log, and drag it across the companionway
to close it. Drag it the other way to open it. That's what our distant ancestors did to secure their caves.
And there has been very little progress from that system...on sailboats, anyway. It's really pretty pathetic. Pay a couple hundred thousand for a sailboat...and you get boards! Yippee.
Yeah, I get that boards will stop a wave better than a door, but there's no reason why a set of stout boards can't be used as a backup in a rough weather
There are no truly elegant solutions to the companionway
door problem. The problem begins at the designer's drafting table. Are any designers thinking outside of the box on this issue?
The Zarcor doors are one of the best retro solutions to a basic design problem. But, when open, the doors protrude into the cockpit
area where all the commotion is while underway. They're fine when not sailing, or when things have at least calmed down.
On some boats, the companionway is recessed, so that it lies 20" or so forward of the adjacent cockpit
bulkheads. Outward swinging doors would be ideal with this arrangement, as when they're swung open, they wouldn't be in the way.
For now, if you want a decent companionway door, get a Searay.