Ever seen a locking bag that could be locked to the bike? This might take a little explaining.

I've got stuff on my bike that removes easily and could be stolen: head light, tail light, cyclometer. If I'm riding to a concert or something, I don't want to lug that kind of stuff around in my pockets, but I don't want to leave it sitting out on my bike for thieves, either.

I've been looking for some kind of small bag or pouch made from a tough material (like those locking bags banks use for deposits) that could be locked to the bike. I'm imagining something with a sturdy strap that would fit through an opening on the other side, so you could then feed your U-lock or chain through the loop, thus locking the bag shut and to the bike at the same time.

Has anyone ever seen anything like this? Is this overkill for a problem with a simpler solution?
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.