I need to leave fairly small-medium window wide open

I have been told about something called 'styrofoam'

can anyone recommend any websites? I have googled it but am a bit
conufsed by the number of different types of foam.


(need to leave a little hole in the corner for an antenna - I can cut
this out myself)
(window is only to be left open for a few hours at a time and opens out
onto private courtyard with CCTV and is high up so no security risk)


I'm hoping someone here can help me! thanks