I got a complete bag assembly for my Electrolux C12, the double-zip job with the ST filltube, and that thing is just as insane as it was with the shakeout bag, and the bags are just the best, they just breathe so well while filtering to a high standard, they're just brilliant...
And be aware, the ST filltube will NOT fit "any" F&G filltubes, they will only fit *genuine* Eureka/Sanitaire F&G tubes, the generics are slimmer and thus the base of the ST tube will not seal properly (I have a generic bag and I have tried it, despite being told before about it, and the ST tube does not stay put), it only works on the real deal...
You may aswell buy the whole bag assembly in strictlybojack's link, it makes the conversion simpler and makes your vac look newer (my C12 looks all nice and new with it's Sanitaire ST bag!!), and installing is easy as pie, just remove the old bag, and fit the new one in it's place...
And a pic of the C12 to show how nice it looks with it's ST bag:
