Ah yes I had those in my childhood home. You probably could rig up a seal to also take care of the “triangles” at the top and bottom (these seals aren’t that great anyway, you mostly just want to create a “good enough” cover), but it would probably be more trouble than it’s worth. More expensive, but ductless mini-split air conditioner will probably be what the UK goes for if/when the average temp starts being unbearably high for fans. They’re very common in parts of Asia and are becoming more common in the US.