Ever seen a locking bag that could be locked to the bike?
Do you mean something like this from PacSafe it's got a secure steel mesh inside:



I remember using one of their bigger backpack bags when travelling to New Zealand a few years ago, and the only complaints I had were from the TSA when we went through the US.

Alternatively if you have a rack on the back, then it may be possible to attach a lockable box to that in some secure way. Although my worry would be that anyone who noticed such a box, would immediately wonder what was locked up inside and decide to break it open.