A bag.

Specifically a bag for a U-lock.

That attaches to the frame like one of those little bags that goes in front of where the downtube and toptube meet.

But shaped like a U-lock.

With a little velcro strap that keeps the U-lock from falling out.

The probem is that every single lock I've bought that has one of those frame attaching thingy's doesn't really fit any frame I've had (and still be able to carry two waterbottles) and if I do get it to work right, it rattles which bugs me.

If I put my lock in my panniers or trunk bag it's a pain to get out. Plust, often all I carry is a lock. Seems silly to carry a pannier or large bag with nothing but a lock in it.

I could also put it in my messenger bag. But that bag is heavy enough.

Or attach it to my rear rack with bungee cords. But then you have the rattling issue.

I want a special bag for my lock. They make them for cell phones which fit in one's pocket. My lock doesn't fit in my pocket.

Never seen one... but definatly a good idea. Not sure if you'd need a whole bag, or just a sling of some sort. Like something that would hold the top bar, and just a strap that goes to the bottom to hold it from swing around.

I got tired of the U-locks bulkyness... that's why I have a coil lock. Wraps around the bars on my BMX, or the seat post of my roadie.

i am in the same trouble because my bike is a dahon folding one

here is an idea or two

make one, but make it look nice and pretty

then make one million, and sell them for lots of money, cause many would want one!

This is where I am. Unfortunately, the key part on mine is two pieces, so it rattles internally unless I can tie down both parts. I had it for a while, using the holder for the shackle and bungying the rest, but it got in the way after I lowered my stem.

A bag would be interesting, especially if you could attach it like a tiny pannier to make sure it's out of the way.

I use velco straps for just the same reason. I don't see the need for a 'bag' per se.

Here's a pic of my setup. I used sticky-back velcro so the straps just stick on the frame. It works really well for me. Hope you find what works for you.

I don't have a u-lock or I would try it - Something like this pump mount might work for you.

PAC Designs makes a U-Lock belt. Unfortunately, the picture on the website is broken, so it shows the key wrist strap instead. I'm sure you could strap it to your frame somehow with little to no modification. I think a couple other messenger bag companies make lock holders too, so you might want to check around.

Go to SS/FG, and look at some of the bag threads. The guys making messenger bags are generally 1-3 man concerns (Chrome et al aside) who would respond well to an idea like this. Bagaboo would probably go for it, but are in Hungary. Maybe Seagull? These guys will be a lot more interested than Ortlieb or Carradice would.

If you have a rack, and are carrying just the lock, here's what you can do:
Open the lock, slip a nice thick rubber band around the shackle, then close the lock.
Hold the lock with the U part pointing down, and slip it between the outer rail of the rack (where the pannier hooks would sit), and the main bed of the rack. The thick part of the lock won't fit through this space, so it will rest there.

Now loop the rubber band over the lower hook (where the bungie of the pannier would attach). This will hold it in place to prevent rattling. The rack struts will keep the lock away from the wheel.

This works with most normal racks (Blackburn, etc). If you have a Tubus Fly or something, I guess you're out of luck.

I like the velcro straps! That's the best idea I've ever seen!

Going home to take all the stupid plastic mounts off.

What I'm envisioning is some kind of holster-type bag that the lock slides into. Easy access is an important feature.

Go to your local Army/Navy military surplus store. They usually stock a whole slew of pouches in various sizes and shapes. You should be able to find something that you can strap to your frame.

I just implemented the velcro straps solution myself, I bought the Velcro brand "one-wraps", 5 for 3 bucks (in NYC). They're multi-colored and ugly, but whatever, they're holding my NY Fughedaboutit beautifully, and no rattling.