Bedroom air conditioner installed! As you can see, I made a wooden adapter to fit a cheap vertical sliding window unit into a horizontally sliding casement window. The foam wrapped bottom simulates a sill lip, and I used the double window to clamp both the unit and a wooden top insert which also braces the unit. The three quarter inch plywood exactly slots into the aluminium frame, and I used a little plug top left to apply pressure to keep the top panel in place. The unit is also screwed onto the plywood just in case, and both sides are stuffed with foam to create an airtight seal and keep traffic noise out. Best of all, none of this required any modification of the landlord's property, and it can all be removed later without leaving a trace.
I think I deserve a beer. Shame we don't have any.