OK, it is not too fancy. 18mm
plywood with screws pre-installed around the edges, window
draft stick on
sealant, and a good screwdriver kept next to it. For the L450 the window holes are square 44cm x 44cm, so I made the
panels 52cm x 52cm. I have marked out the location to fit if necessary. I have not drilled into the window frame the
wood is soft enough for the screws to grip and go in OK. I also have a
power drill with a bit handy to drive the screws in.