This time I used the relief design of an artwork was creating in 2016. The oak wood of the artwork started to warp, so I did not finish it then. Now, the blocks are glued on okoumé ply wood, which is unlikely to warp.
The structure has a 5 mm height on the 'inside' and a 9 mm height around the 5 mm blocks. This results in a nice transition between 9 and 5.