Yeah, that would work even better at deflecting heat - there's a reason chemists and physicists use simple aluminium foil as make-shift insulators (might also block some the neighbour's WiFi signal, boosting the reception of your own, but that's a bit of a tin-foil hypothesis :P).

You'd miss on the evaporation cooling though. Maybe a two-layer approach would work best of all low-tech hacks! :)

In the past, when I was living in an apartment with a wider windowsill, I used to put a reflector made from cardboard covered with aluminium foil on the inside and sun-hungry plants inbetween, creating a miniature greenhouse. Open windows were a requirement of course, because it would still trap heat otherwise.