Usual approach, make the box as one entity pinning and gluing the top and bottom to the already fixed sides making a sealed box, sand all faces to a good finish, then saw apart making a lid and base that you can guarantee will match each other even if very slightly out of square.
Worked very well until we get to the sanding stage where I find I've been heavy handed and sanded through the top ply - surely NOT - 11.45 mm thick (measured) 7 ply each ply should be about 1.6 mm thick - no way have I sanded 1.6 mm!
. . so I did an experiment with a scrap of the ply, and sanded until I got the same effect then measured how much I had removed. Answer a bit less than SIX thou so what's going on ????
It seems that it's NOT 7 ply but 9 ply, the two outer 'plys' being only 6 thou thick and basically just a colour layer that is paper thin.
. . .what junk is being foisted onto us these days !!!!