I saw this on YouTube the other day. Storage chests embedded into & laying flush with the floor level that you could walk upon.
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.
Remove two foundations
Then use fence foundations to go down two levels in the usual way.
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.
Start placing. This can work without gaps with just two fence foundations running parallel to the sides of the chests.
There’ll be around a 1-2cm external border with them just overlapping in the center a smidge.
You can place things on top, however your mileage will vary on what and which angle it succeeds.
It takes a bit of getting used to but you’re moving around looking at a downward angle anyway.
Note: I tried Eldarium reinforced chests but they start flickering rapidly as soon as they’re enclosed and you stand back a certain distance.