It's not directly, one can sit in a ちょこん fashion - that is, small and quietly. It's an adverb, not an onomatopoeia.
"Onomatopoeia" was a poor word choice on my part, yes - it's more an 'unsound effect' like "jiiiiii" for staring. I wonder if Japanese has a word for this sort of thing.
Edit: according to the very TV Tropes page I linked, it's "gitaigo".
Funny, of my current cats, the female thinks of cardboard boxes not as ready-made homes, but as chewing practice. I think she thinks she's a bird, and wants to build a nest, or something, because she loves nothing more than shredding cardboard to bits, then rolling around in the shreds.
