…is hard.
I started off digging through the pile of scrap Tongue and Groove boards because I knew that the boards I cut out of the opening should exist somewhere. Then I re-arranged them into a door. Parts I didn’t still have were easily reformed from scrap.
The plan was for 2 layers. One from the original pieces that would fit into the opening like a puzzle piece. The 2nd layer would be wider than the 1st would lay over it on the outside. So… like a Cap and a Plug, I guess.
So here’s the plug from the original pieces. We carried it over to the opening to make sure it fit.

Then we laid out some T&G boards and set the plug on the cap, and screwed the plug to the cap with short screws.

We realized that because of the T&G, that we hadn’t actually secured the door. While screwed together top to bottom, it could still pull apart left to right. So we attached vertical pieces to hold it all together, making sure we overlapped boards.

Then I think we just used the jig saw’s guide as the offset to cut out the plug.

Here’s the other side.

So yeah, the structure was done. We then set the door in place where we liked it, and screwed it to the opening to hold it in place. Effectively making a box again. From there we needed a frame to match the offset of the Cap so the hinges could sit flush.

We needed 4 since it was going to be a dutch door at some point. Then out came the screws holding it in place… and a moment of truth.

It stayed… it didn’t fall off…

And it opened! HUZZAH!
However now it had a pretty significant sag. Which we expected to some extent. Gates sag on their own, but then add in the fact that the rails were slotted against themselves and can slide around. Pretty standard gate framing on the bottom came next. However the top required some creative thinking because we plan on putting a window in it, which means we can’t put a cross piece through the middle.

Not actually happy with how the upper turned out, and while solid, the hinges have wiggle so we still have sag. I have a plan, though. Next up came hardware!

The top hasp provides a lock to keep people out. The slide bolt attaches the top to the bottom, and that ring is actually a pass through to the outside lock.

Realized that lock wouldn’t keep the door secure when driving down the road, and didn’t actually provide security. So this keyed lock was added. Sadly, because of the natural contraction and expansion of wood, that lock doesn’t always fit smoothly, but it works!

We still had some sag in the door, so I added another Z pattern on the inside. It classes it up a bit, too.

The bottom door is now VERY solid… but off by a quarter inch. Sigh. It works, so to make it perfect later, I’ll save off part of the lower threshold and add a drip edge. For now this is fine.