I placed black self-adhesive draught-excluding tape on the wall around the outside of the window. In my case there was a window sill, so I had to screw a baton of 10mm x 44mm cross section wood onto the windowsill so that it was flush with the wall to allow me to place the draught excluding tape all around the window, all flush to one surface. I hold a piece of 10mm thick plywood against the draught excluding tape with 6 bolts that protrude from the wall through holes in the ply. Importantly, the holes should be outside the draught excluding tape, so that light from the window can't seep through the holes. I use 6 nuts to compress the plywood against the draught excluding tape - it compresses down from about 10mm (uncompressed) to about 5 mm when compressed. This works very well and the ply can easily be removed (you could secure it with butterfly nuts so you don't need a spanner to remove it).