I found this excellent video for making your own cat perch from wood to specifically fit your size window. It makes it look really simple... of course, nothing is really simple, right? There are many other options out there both to make or to purchase. A quick Google search will find you many different styles.
Here are a few styles that are available commercially,
but if you're handy, just build your own cat perch.
Cat Window Perch Ideas
Send me a picture of your finished project and I'll post it.
Thanks! --Lori Lee