Can I cover old brick vent hole which does not close?

I've just moved into a rented flat and the front room doesn't have central heating, just one electric heater. The rest of the house does, I don't know why this is a design choice from the landlord. We've noticed that this room is NOTICEABLY colder than the rest of the property. At first I thought it was due to 2 large double glazed balcony doors. After some birds outside caused a ruckus and we were able to hear it clearly INSIDE the flat, I've since found we have a vent hole which seemingly can't be shut and leads straight outside into a brick vent. I get that it's for airflow, but it makes the room SO cold. During summer it might be nice but Winter is unbearable. Typically landlord has gone overseas for Xmas, is there anything I can do to cover this even temporarily? As I can't even take off the facia, I'm thinking maybe a velcro strip along all edges with some material or a carpet offcut? As its rented I can't actually do any work on it (plus I kinda think the landlord should really do this)