troutlily's Profile

i have similar window sills. i've gone with 2 methods. you can either buy big L-shaped metal angles that screw in to the 3" exterior vertical portion of the sill you've drawn, and set the box on them. there are weight limits so don't go crazy. i've had a pretty hearty jade plant outside my window on 2 of these angles for over a year now and they haven't budged. you have to crane yourself outside the window to screw back in toward the house, so don't fall out!
secondly, you can also screw smaller metal angles into the side of the window box, set it on the 5" horizontal portion you've drawn, and screw the metal angles direclty into this 5" portion. since the 5" area is sloped away from the house, you might want to bend the angles slightly if you want the box perfectly vertical but last i checked plants can grow on hills. in this case the box is actually sitting on the sill and depending on how tall it is will stick up in front of the window. the former version, the box is hung sort of below the window. my landlord made me take mine down that were over a street (liability) but in your case, in an alley should be ok. alley cats lookout!