cutting a hole in polycarbonate

My new potting shed is up and functional. One thing we didn't make adequate provision for when we designed it was ventilation. I can already tell that it is going to get too hot in there. In the summer we will be away for an extended period so I need to get some ventilation sorted. I have got an automatic greenhouse vent opener. The best way that I can come up with is to cut an opening section in the polycarbonate window and bolt the opener to it, creating a vent.
Can anyone advise me on the neatest way to cut a square into the 5mm polycarbonate? I was thinking a really fine saw blade might do it but unsure on how to start it off....any thoughts? thanks
Phil

Thanks, I'll try that

best thing to do is take it out and clamp between 2 pieces of wood . you need to take away the flexibility

its also worth putting tape over the cut line as this will stop the foot of the jigsaw marking the polycarbonate.