What you see is two thirds of my hatch.
I read about many who were able to store their one piece hatch under their helm seat. I tried many approaches to this, tilted different ways, even trimmed off some of the upper corners; but I could never get mine to fit. It seems like there is a fair bit of size variability in the under seat area.
I quickly got tired of having to find a place to store the large panel inside so I cut it into thirds like normal sailboat washboards. I used matching aluminum strips to cover the joints. This made storage much easier.

Later, now that I had some smaller pieces I re-visited the under the seat idea. The angle on the hatchboards is different than the angle on the transom sides so I made some beveled fillers from wood with a groove in it to match the hatchboards shape. This lets me drop in the two largest panels under the helm seat. It does not go all the way to the floor, there is still a gap. Early on I had put some shock cord across this area and I just left it to keep the big stuff in place. The third smallest panel can also be stored under the seat velcroed to the other two.
These pictures aren't the best, the boards fit better than it looks I just didn't pay enough attention before I clicked the shutter.
I also have a fabric panel that came with my Dowsar enclosure that closes in this opening. It does go all the way to the floor sealing the opening. It and the hatchboards can actually both be in place under the seat at the same time. It fits inside the hard panels.
I added fasteners for the fabric panel at the companionway as well. This lets me choose the hard or the soft panel at either end of the cockpit. Most of the time we have the hard hatchboards stored aft under the seat. There are times, at marina's mainly, when we may want some privacy at the companionway but don't want to mess with the hard boards. I can just snap up the fabric transom panel to screen off the cabin quickly without getting the hard panels out.
The fabric panel is also good when you want to seal up some but not all the way. On our first night out a few weeks ago we put in the hard hatch boards and closed the sliding hatch. It was cold and we had our 110v heater turned to low. In the morning we had quite a bit of condensation inside from the 5 of us breathing all night.
On the next two nights I replaced the hard hatchboards with the fabric panel at the companionway. I still slid the top hatch closed. This closed up the cabin enough to keep it nice and comfortable with the heater yet let enough air move that there was no condensation either morning.