If the original cap isn’t available, the safest short-term solution is to make a protective cover that fits over the built-in hood without touching the glass. People commonly improvise one from cardboard and gaffer tape, or from a piece of plastic pipe of the right diameter. The key is that it should grip the hood securely and leave clearance in front of the bulbous element.

For a better long-term fix, look for an original replacement cap from a Samyang/Rokinon parts seller, since these lenses are often sold under both names. Another practical option is a 3D-printed replacement if you can find a suitable design or someone who can model one.

Whatever you use, make sure the cover is rigid enough to resist pressure in a bag and doesn’t shed debris onto the lens.