ventilation after a gas fire has been removed

[scorchy]

i am trying to find a small sliding vent to put in a internal wall where i have taken a gas fire out,
i have been advised by some friends that this is needed to be done to stop condensation coming down the gas flume and making the wall damp,
the smallest vent i can find is a sliding 3" x 7" but i was wondering if you can get smaller.has anyone got any ideas,
thanks scorchy