Inconspicauous, closable ventilators other than trickle vents?

I'm decorating a room where the fireplace was closed up many years ago without any ventilation. There are no signs of damp in the chimney; and the risks of it should be reduced further now we've had the chimney capped with a "pepper pot" vent. But I'd like to provide a bit of ventilation from the room to guard against condensation in the chimney. Also the windows have no trickle vents (and being aluminium framed I can't fit any). I'd prefer a vent we can close when it's parky and blowing a gale. Something like a round soffit vent in the skirting board might be OK but I can't find any which are closable. I don't fancy a plastic hit-and-miss in the wall or skirting board (we're not houseproud but there are limits to what 'er indoors will stick). All I've found so far are trickle vents for windows which I think I could fit to the skirting boards by just drilling a line of holes. (I'm not up to router work.) Any other ideas please?