Hi Steve,
The stringers aren't 12mm all the way. It is for optical and mounting use 12mm.
At the bottom where the bottom meets the tunnelside it is only 4mm.
same where the tunnel sheeting is glued, this is also 4mm.
The middle stringer is to create the ridge in the tunnel, that is only for optical purposes
And will make it hardly any heavier.
But makes the final cat look much better.