the depth is up to you but keep in mind how large your biggest ribbon spools are. my big ones here are probably about 7" so my box needs to be at least 7" deep.
hold your largest ribbon spool up on the inside wall of your box, making sure to give yourself about an inch on the top and back, and mark where you want the hole to go. measure down on the other side and mark it. i measured mine to stick about 1/4" out the front of the box to make sure the light could hit.
drill your holes just slightly larger than the diameter of your dowel for easier spool replacement. and assemble.
then cap the ends with old doorknobs, water spicket handles, large wooden beads or just about anything that has an existing hole or that you can drill a hole into. keep in mind that you will need to keep one end removable for changing out your ribbon.
i affixed my box to the wall but you could also place it on a shelf or counter. perfect for your sewing room or space. so many options!! i'd love to see what you come up with if you do one for yourself. or if i forgot something and you have questions please feel free to ask.
check out Janne's colorful version she made after seeing mine when she came to visit a few months back.