I'd argue that having items that give you several Decanic bonuses isn't "cheating" at all - it's actually embracing the whole point of the system, which is that mages who cast in the Cabal style will have a whole bunch of weird occult items on them. Any convenience benefit for having one item that gives a bunch of bonuses is generally going to be countered by the fact that that item is almost certainly going to be remarkable. Anyone might have a stick of cinnamon gum, a flashlight, and a stick of laurelwood incense on them, making it easier to cast a Light and Darkness spell. Happening to have a a lozenge pendant divided into four alternate black and white squares, made of alternating laurel and hazelwood, framed in applewood painted yellow and set with carnelians, all on a gold chain on you, well, that's going to stand out a lot more.
Incidently, there's an old Pyramid article that's still available as a sample online, "
Decanic Focusers", that provides objects for all the decans. If you do end up letting people do this in your game, it's an excellent resource for how the various objects might look. I got the example object I used for Phoubêl, above, from there.