Run some wire around the perimeter of the window held in place with
small suction cup hooks. The length isn't critical, but as DxAce said
too much antenna and you will flood the receiver with signals. A wire
antenna isn't particularly selective, and it will pick up all signals
more or less equally. So Radio Havana will come in nice and strong,
but so will static crashes and hash from a nearby computer or tv.

Modern portable receivers are quite sensitive and they pick up a lot
with just the built-in whip antenna. Unless you are inside a steel
frame building that radio should hear lots of shortwave broadcasts and
ham stations with just the whip.