I don't think the sterile zone has to be big at all - we've just not had a need for it to be any smaller for a while. One block either side pretty much:
https://www.youtube.com/watch?v=GUdKYfLHqxk
A few hundred packets of 50?
Big risk buying online today - if they don't arrive tomorrow then it's going to be too late.
Also: I think I'm a bit colour bling between bright yellows and greens, but isn't that the wrong colour?