The Weaver's Guild....
You know the weave and such.....adds to thier secrecy.
Plus they could use a building fronted by a clothier or adjacent to a Rug makers shop as a plausable cover.
You could even come up with a secret password for them to enter the cabal. Say....Speaking the Phrase "Oh I know many things about weaving", in response to the challenge "My lord/lady, this is a fine piece" as the shopkeeper acknowledges thier choice of a certain piece.
Just a quick thought the entry code needs more work though.