Do NOT use fiberglass insulation.
Fiberglass doesn't stop airflow, just slows it down. And using it to close off a vent, will lead to a lot of airborne fibers - which is really really really bad for you...
Take off the cover, cover the backside with plastic or a cut-to-size piece of cardboard, or duct tape, or... something solid / not porous.
If air still leaks around the edges, use some peel & stick foam weatherstripping.