Here is a photo of one of the window shelves I created for my cats, Jack and Stormy. The only place in the house where they were able to access a window view was the loft. I decided, after dealing with hairballs on my bed (and worse), that the loft would be off-limits during the day. So, I needed to provide the cats with some kitty TV! The shelves are supported by the inverted bracket at one end, and a brace running the length underneath. The bracket has wingnuts holding it and there is only one screw running down through the shelf to the brace below, that way, I can quickly remove the shelf if needed. I used 2-sided carpet tape to attach some carpet to the shelves for comfort and clawing. The shelf above is in the office.
Stormy is sitting on the shelf in the great room. Both shelves are trapezoids which give the cats plenty of sleeping space, but allow sufficient room for me to access the bathroom, or, in the office, my desk. I was quite pleased with how much weight the shelves can bear with such low profile support. They are a great place to snooze!