I was driving w/sw on tx 130 south about 31 miles north of i10. it was partially cloudy with a lot of blue sky. i saw a rectangle up in the sky at about 50 degrees up and about 10 degrees to the right. the sun was about 20 degrees more to the right. the rectangle was a perfect rectangle with good 90 degree corners and straight lines for the sides. it looked like a window in the sky. it was grayish and did not move. when a light cloud passed in front of it, it then seemed to glow very bright white thru the cloud. a few seconds later a darker cloud passed in front of it and it disappeared. with the wavy road and the fact of driving an rv at 75 miles per hour i lost track of it. there were 3 other people in the car and i yelled at them to look but they were all asleep. it reminded me of a scene in the movie "the truman show" where he lived in a dome that looked like the sky and a panel fell off leaving a rectangular hole in the sky. i have watched planes for years and have never seen anything like this before.