To translate this as "coussinet" seems, on one level, like a cop-out, but from another perspective I think it's a prudent solution to an intractable problem, since it's very difficult to tell exactly what the author means by the term. However, I do think there's a strong chance that his use of it is inspired by, or based on, one of its established architectural meanings, which is:

"Partie renflée dans un chapiteau ionique, formant le milieu de la bande qui, des deux côtés de la colonne, s'enroule en volute."
http://www.cnrtl.fr/definition/coussinet

With this meaning the French term is also used in English, though according to the Oxford Dictionary of Architecture it refers in English to the volutes as well as the bands between:

"Cushion of the Ionic capital, including their two volutes and their connecting bands, like a rolled mattress"
https://books.google.es/books?id=4Lu6BwAAQBAJ&pg=PA206#v=one...

And according to Joseph Gwilt in 1826:

"The word Coussinet is also used for the ornament in the Ionic Capital between the abacus and the echinus or quarter round, which serves to form the volute. It is thus named from its representing a cushion or pillow pressed by the weight over it and bound with the strap or girdle"
Rudiments of architecture, practical and theoretical, with plates
https://books.google.es/books?id=kbcaAAAAYAAJ&pg=PA193&lpg=P...

Well, possibly our author's use of the word was based on the resemblance between the top of the handle, into which the mirror was inserted, and the capital at the top of a column (he does use the words "colonnette" and "chapiteau"). In the wordreference-com discussion, although the pictures are very tiny there do seem to be elements somewhat like Ionic volutes.

So really I agree with Frances S about the base meaning that probably lies behind this term. However, given that the author is adapting it to a different context, I think it would be better to use "coussinet", which has this architectural meaning in English but is also open to wider interpretation to fit the specific context. It is quite likely that the author is coining a term to denote an element that has no existing name in English (I certainly can't find one).