What a cute space! I’d do a nice stationary pouf - I actually found those a lot more convenient for extended rocking when you end up nap trapped (my daughter was a motion junkie who would only sleep on the move).

I was also going to recommend a pouf. That's what my daughter used with her Pottery Barn chair (just like yours.)

I recently ordered an 18 x 18 pouf cover from Amazon. You fill it with old towels, or pillows (I am using bed pillows.) Look up "Boho Tufted Decorative Unstuffed Pouf - Farmhouse Casual Ottoman Pouf Cover".

I think your nursery needs some color. Read THIS article.