Don't be hacking about that K0 of yours mate, it's worth more than the average K, but not as much as a "sandcast" (early 1969) K0.
There is no recommended procedure for carrying a spare helmet on a bike, the recommendations above (apart, perhaps Raul's elastic net thing)are all fraught with danger, so I just wouldn't do it unless I had a pannier or top box to securely carry it.
I don't like helmet locks anyway, I've know of pissed (drunk)Aussies using peoples helmets for urinals, ha ha!:)