If you're feeling brave you can mask off and cut out a vent using a small cutting disc or drill a pilot hole and get busy with the jigsaw. File down the edges, primer, coat of your body colour and bob, is your mothers brother, wing ventage. Did it on my mates mk1 mini, looked mint and noone else had one like it. Failing that get one from the scrappies to experiment on before attacking your bodywork.