Fortunately, as fortune does favour the foolish, the inner sides of the covers each had a small moulding mark in the centre, however vague. I then took my mini drill-press with a carbide drill-bit and drilled them out slightly for a compass leg-tip to seat...
Incidentally, the magnet for the blank cover can be off-centre, if one is cross-eyed, but the magnet for the printed cover must be dead-centre, thus requiring a right good rap onto the head.
The risers for the magnets, and from a 1mm-thick aluminium scrap-sheet...
The risers were then cut out with "tin snips", and dressed...
The two at far right will be epoxied together, and for the blank cover.