I agree with the idea of making them all 12" using borders of a solid fabric - the same fabric for all - navy maybe or the wood grain GloBug suggested.
Sash with grey solid.
Border with either navy or grey, or both thereby having 2 borders - one narrower than the other, outer border being the wider of the 2..
I would not use any patterned fabric - they would detract from your pictures.
Great job; he'll love it.