This is the cover that goes over the 掛け布団.
The English will depend on your target market, as there are various regional differences for the naming.
Do you need US English?

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Note added at 12 mins (2007-08-13 23:07:39 GMT)
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Main Entry: quilt
Part of Speech: noun
Definition: bed covering
Synonyms: bedspread, comforter, cover, coverlet, down, duvet, eiderdown, continental quilt
Source: Roget's New Millennium™ Thesaurus, First Edition (v 1.3.1)

# 掛け布団
quilt // top cover

# 掛け布団の長さに合わせて脚を伸ばせ。
Stretch your legs according to your coverlet.《諺》

# 掛け布団をあごまでかけて
with the covers up to one's chin

# 掛け布団をあごまでかけてベッドに入る
get into bed with the covers up to one's chin

# 掛け布団カバー
comforter cover
http://eow.alc.co.jp/掛け布団/UTF-8/

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Note added at 14 mins (2007-08-13 23:09:02 GMT)
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度々すみません。
Perhaps "comforter cover" or "quilt cover" would be the most universally understood.