So I've found this cardboard box, seems about the right size.
I cut down the edges of it to open it up. The two smaller pieces are still attached by tape. First I remove them.
Then I put the bigger part of the box in the window to measure up for size, and tape one of the end pieces onto it in the right place.
The other end piece will be the bottom of my thing. I tape it in place so that the fold in the card lines up with the bottom of the box.
Now I want to make it box shaped again. I find another piece of card. I fold it in half both ways and cut a slit halfway across the shorter line.
I fold the whole thing together liker so, and I cut off the excess. I use it (and some more tape) to support the edge of the box.
I make a mirror corner and tape the other edge to that. Then I tape the top of the box.
Notice how I've left the bottom part loose? That's part of the plan. Just need to trim the edge off that.
Now, if we put the bottom flap on the windowsill, and sit Montgomery on top of it, the thing will stay put. Then to close it, I just have to fold it upwards. Voila!
Yay! Now nobody can see in if I don't want them to and I won't get glare on the TV anymore :) So it works, but it's not very pretty. You know how I like things to be pretty, right?
I'll wrap the thing up in pretty coloured paper.