Mispickel has her own chair in the kitchen. Theoretically, it's supposed to keep her from stealing our chairs. That doesn't work, of course, because she's a cat and they never want anything that wasn't already yours, but she does like to sit in her own chair and watch the birds at the feeder in the backyard. Cat television. (She's indoor only. The birds are in no danger, at least not from her, but they're entertaining.)
We've had an old fleece blanket folded up on the seat as a cushion, but it slides off all the time. Sometimes she sleeps on the blanket on the floor, which seems kind of pathetic.
So, for Christmas, I got her a cat bed and then made a cover for it that could be tied to the chair to keep it in place. It's washable, too, of course.
The step-by-step Flickr set is here. The basic idea is to cut one circle--actually two overlapping panels--for the bottom, and then a larger circle for the top, and gather the top onto the bottom with straps set into the seam. It took me about an hour and a half, some scrap fabric and old bias trim for the bottom and straps, and a minkee remnant from Joann's for the top.
And, of course, it could be made without the straps if you just wanted a cover for your cat bed.