Window seats

I want to buy Milo a window seat. My childhood cat Katie had one and loved it. Milo loves to sit on our window sill and near by chairs to bird stalk so I figured this would be a great thing to buy. We tried going to all the local stores like Pet Smart, PetCo, and the like. They all have window seats, but they all are weird. I know I was little when I had Katie, but her window seat was different. If I remember correctly it just attached to the window with some brackets or something. There was no screws or anything needed. All the ones I have found either require some sort of nails or screws screwed into the window that will make holes in the window sill or have a strip of velcro to attach it which will mess up the paint. Why can't I find any normal window seats?