Hi I meant sandwich as used in board construction, or planes or other applications :
-high density pvc or pet foam ( like airex 70 to 100 kg / m3 ) - say 15mm to 20mm thick
the high density foam core with the skins creates a beam effect,
the stiffness increases with the cube of the core thickness for a given skin carbon layup.
Overall it saves weight because a given stiffness can be achieved by spacing out two minimal layups of fiber,
rather than increasing the number of layers without any core spacing.
that saves weight because pvc foam density is substantially lower than the one of carbon+epoxy layup.
Just a thought for your next project maybe
It seams like you have achieved a very nice result anyway with your build with the stiffeners under congrats
I am not in in position to criticize board weight, I would need to loose 10kg myself first