A raised crust, like the lid of a basket. Hence Shakespeare speaks of a “custard coffin” (Taming of the Shrew, iv. 3. Mahomet's Coffin)
“Of the paste a coffin will I rear.'”
Shakespeare: Titus Andronicus, v. 2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894