I don't have a window sill in my dorm room, but I wanted my plant friends to be able to enjoy light in the window, so I picked up a board and some hardware and made them a simple shelf that attaches to the window without any holes.
Chose my hardware purely based on what they had at Lowes, but if it helps...
Bolts are 3/8" X 6"
Board is 1/4" X 5 1/2" X 2'
The picture with the hardware in a line is the order I put it together in. You may need something a bit different depending on your window.