Magnetic Fridge Pen Holder
This is a magnetic fridge pen holder capable of holding up to 5 writing instruments (up to 12mm in diameter). All you need are two magnets (ideally 30x30mm) which you can superglue to the back.
Please note that what you see in the pictures does not match the .stl file exactly. The .stl file has since been modified to make it better (changed height from 30mm to 50mm, cylinder diameter from 10mm to 12mm made walls thinner).
UPDATE: If you're finding the walls of v1 are too thin, v2 fixes this problem by increasing the thickness. Better to print v2 to be on the safe side.