The plan for the spar for the mini is to make a “box” with ply on both sides of foam that will go from one side to the other of the mast and some UD carbon on the inside where the box meet the sides.
On the outside UD carbon and BX glass or carbon. There should be no problem as the fibres are not in the same direction.
With the rather small cross section of the F16 spar I don’t think I need to make transversal walls.