Just a quick update. I thought about a solutions today and tonight I tried it. I got a piece of pizza carton and cut it in the right shape.
Unlike the masking tape this gives me the possibility to adjust and then taping it to the right area.
The only thing is I need to be nice and straight on the airbrush because the masking properties of the carton aren’t too great.. then again, if it does actually leak through I won’t mind too much about doing another layer of the light blue. The mottling is not perfect and I wouldn’t mind another go at it should it be necessary.