This is my idea for the spool rack.
Using these 20mm clamps
I'll put a piece of 20mm aluminium tube about 200mm long centres in the clamp. Then a reel goes either side of the clamp. I'll have three clamps so can store six reels. The front four reels will all align with the spool entry hole in the top of the cabinet, allowing easy switching between four material colours.
I've printed clamps to stop the reels faking off. You can buy these in aluminium or steel but they are expensive!