Oh OK. But you're right; a baggy would most definitely work. And I think it would help to ensure the ends don't dry at all. Ends that don't dry=ends that don't break=retention=length.
Ever read Yvette's journey and how keeping her hair ends moist led to dream hair? Such an inspiring story. You can see it here.
Keep it coming ladies. OP your bun came out great! Why did you braid the nape?