Build a box that is bigger than the hole. Like if the hole is 4'' diameter, the size and shape of a shoebox is good.

Now paint the inside matte black.

Install a baffle down most of the length as shown.

You want to bend the light around a corner because light CAN'T bend around corners, see? You get ventilation without light leakage.

You biult a grow box you can probably figure out how to build this baffle.