Many thanks for your replies and sorry for the slow response… life tends to get in the way of woodworking 🙂
I like the rebate idea, this would also reduce the immense weight of the lid a tiny bit. Also, climate control and hiding any irregularities and gaps seem important functions of the dust seal.
However, I wonder: would a thin layer of rubber or felt around the top not achieve the same? The overbuilt lid would compress the rubber/felt and thus create a pretty tight seal that would also hide any gaps (?). The chest would only live indoors.
For various reasons, adding a dust seal later would either be very labour intensive and/or look ugly. Thus, I would prefer to bite the bullet now if it’s inevitable.
I would much appreciate it if you have any further thoughts.