You may be as well getting one of those plastic sheets for the lid.
You need to move the lid quite a lot and those windows are heavy. Fine for the sides, back and front though.
I suppose you could use two at the back two at the front and one at each side that would give you a 10X5X1 box. The other two could go on top at each end that would leave you a 8X5 area to put a lid on.
If you get a strip of plastic sheeting long enough, you could fix it with nails to the top of the back of the box. Then bring it forward so that it reaches the ground and weight the sheet with bricks or stones.
The biggest problem will be the puddle on the top when it rains heavily. There is perforated plastic sheeting available which will allow rain in, but still keeps the warmth in too.