Best Window Seat?

Cheeks is going crazy trying to watch the little birds who appear to
be trying to build a nest at the top of my office window. But the sill
is too shallow for her to sit comfortably so she has to stretch on tip
toe from the filing cabinet to watch them.

Any tips on good window seats for kitties that are simple to install
and lightweight and nice for the cat? I expect to go to PetSmart or
elsewhere later today and I would appreciate any advice. It is a
regular sized window. Thanks!