These cat window shelves are Ikea hacks, just regular shelves with a nice soft bed on top. Would be a good idea to put Velcro between the bed and the shelf to help keep it in place. And never mind the link, it doesn't lead anywhere of relevance.
Tree shelves - inspiration for cat tree house. Lovely way to make the cat shelves part of the mural! Also would paint brackets to blend in wall, & consider using less visible brackets that would still support cat weights!