Windowsill holder for dog door and beam

Bet no one but me can use this design, but, here it is anyway. I've got a dog door, and since my cat is a master escape artist and needs to be kept in at night, I have had to build a contraption of 2x4s and brackets to hold it firmly in place when we close it at night. Both parts sit on a nearby windowsill when the door is open, and are prone to falling over, so I built this holder I could screw down to the windowsill to provide an easy place to set them where they won't topple.