Exploding Foci....I never thought of that...
Well I just thought of other Focus items for the incognito sorcerer/wizard going off the "Wrist watch" idea.
Electronics. I'm not talking about magitech or some crap...I mean a focus or enchanted item in the shape, size, and appearance of a common piece of electronics. Because an electronic at its core is just a bunch of metal, wires and circuitboards inside of a bunch of metal and/or plastic. Ti blend, these items don't need to be working electronics, they just need to look the part.
Let's take a Cellphone. you could get like 2 slots out of that easy, 3-4 if you made it out of a cheap, bigger knockoff phone. The 80's "Portable Phone" could make for a lasting rod easy, though it would stand out unless you passed it off as "Retro" by using a Hot Topic Sticker. It doesn't need to be able to make calls, it just needs to look like a phone. Take a dead phone, rip the guts out of it with a screwdriver, make with the magic, reseal, voila!
Laptops could be made into Foci too, since they can get big and cumbersome. Rip the guts out of an older one or one of the "Semi-rugged" computers the Army and Police use that can survive after getting run over by a car...multiple times. You'd have a focus that no one would look at twice and if you were familiar with technology before you got your power back, it would be related.
Hell...make something out of a portable gaming system. A Summoner with her trusty Gameboy to someone Fairies like they're Pokemon...yarg!
You want other incoginto foci, look around your area right this minute. Book, chair, shoes, beer mug, cooking supplies.