What about using a dremel?
I had the idea of similars blocks and my thought was to build them using a dremel or a dremel-like tool and a reinforced cutting disk having the dremel in a fixed stand and a guide to move the blokc inside a thight fitting slot and advance the whole thing sideways a fixed amount (channel + fin thickness) each time you completed a slot.
I just measured the thickness of reinforced disks cuts and it is 1mm and a 5mm deep slot should not be so difficult.