In the video the author left some rather large gaps, which I was able to eliminate. This could a be well-known method for all I know, but I certainly wasn’t aware of it and it’s helped me considerably.
After adding the second block, remove the lowered fence foundation, fill in the first space then border the sunken area with fence foundations that align with the floor level.
I might have noted I overlapped them as much as possible vertically then came back a centimeter. If you already have fence foundations at either end you can also add one between the blocks to make a tighter fit. Here’s a pic of the overlap I had with a line of 4:
I like that setup with the roofed edges, are they surrounding the supports? I had never thought of doing that. Next time I have a base with a central support I’m going to.
Here’s another way using pillars to hide the gaps and inward sloping of stacked chests (pic is low res)