French to English Other Cosmetics, Beauty

It really ought to be simple (and it probably is, only I'm very tired), but I just can't work out what "canon" means here. I have a short article on a marabout in Morocco, where young girls who have been unlucky in love visit the tomb of a saint in the hope of having their wishes granted. There are various rituals they must perform, such as washing, repeating incantations etc., and then this:

Le rituel doit se terminer par le henné. Libre aux filles de choisir la façon d’orner leurs mains. Une femme professionnelle "nekkacha" est habituée à faire ce travail. Quelques-unes préfèrent plutôt étaler le henné sur un *canon*. Elles pressent ensuite avec leurs pieds un citron. L’une d’elles se met à l’œuvre avec ardeur."

What are they spreading henna on? I've gone through all the meanings given by my dictionaries and fail to see the relevance of any of them:

1. A cannon (what would one of those be doing in a cemetery?)
2. Barrel of a gun (what on earth for?!)
3. Spout of a watering can (ditto)
4. Cannonbone of e.g. a horse (not inconceivable, but why that particular bone?)
5. Canion (apparently an obsolete term for some kind of item of clothing)
6. Glass of wine (a little unpalatable, surely?)
7. Model/perfect example (abstract)
8. Attractive young woman (why attractive?)
9. Canon in the musical/religious senses (I think not)

Or is it something else? Or a typo?