We lived in a temp apartment last summer which had a floor-standing rolling unit that vented with one of those flexible 15cm diam hoses hung out a window. It cooled a living room but very very slowly during the hot 2-3 weeks. Worked better in a small bedroom. (Even with a window slightly open to put the hose out.)Yes, exactly why I thought a smaller sized floor unit would work. They're cumbersome but unfortunately we sweat a lot in summer and I would be lying if I said "we'll get used to it". I know we won't but it's not a good enough reason to not move there.
I'd say in Paris there were between 4 and 6 weeks last year where at least a fan was necessary and an air conditioner would've been quite nice.
When I did some research last year most of what I found was rolls of netting meant to be stretched between hooks that you screw into the façade outside the window. Totally impractical if you also want to, say, be able to go outside and sit on your terrace sometimes. Possibly compatible with metal shutters, though.(...screens for cats in windows...)
Called "Filet de protection pour chat", e.g.:
Filet de protection pour chat Trixie, vert olive - À prix avantageux chez zooplus