Hello Frederik,
The companionway is similar to the one I had on my Sadler 25, ~1977-1989. The aim there was to get more headroom in a small boat, as well as providing a sheltered watchkeeping position. It worked well for coastal cruising.
I hope you can see on image 4 that there is an outwards facing lip on the sloping surface and on the after face of the bulkhead, that together form a drainage channel on each side. The sloping forward face and triangular sides of the part above deck level would be of framed glass or acrylic.
For this SibLing, I would make a single plywood washboard that is hinged on its lower edge and has framing on its sides and lower edge to overlap the lip. It would have non-slip on its inner face, as it's to be walked on. I would make the sloping hatch from a sheet of smoked acrylic, hinged on its top edge and with framing on its sides and top edge to overlap the lip. There might be a foam rubber seal around these edges, but I don't think that's essential. The lower edge of the hatch should overlap the washboard by enough to allow drips of water to run off outside, but there need not be a watertight seal here. In fact a small gap is useful for ventilation, so long as it only passes air, not water. There might be a piece of framing on the lower edge that extends upwards so as to cause water to run off at the corners, into the drainage channels.
For a boat to go to the Southern Ocean, or to emulate Mingming's exploits, there would be no washboard, the bulkhead would extend over this area, and a Lewmar 70 hatch, or similar, would be mounted on the sloping face. But I would find that very inconvenient for less demanding cruising and living aboard. My view is that a companionway only needs to be watertight when a full rollover is a real possibility; a knockdown that puts the mast just below horizontal doesn't result in serious flooding, just some inconvenient splashes (how do I know? ;-) )