Trickle vent interior grilles (Slinfold RH13)

I have 2 trickle vent interior grilles for window background ventilation. These are the fixed non-closable type, for a room with a gas appliance or fire, for example. They are white PVC with the fixing holes 386mm apart. The trickle vent slot needs to be 350mm x 12mm to fit the grille inside.