I have a bike, and I had a U-Lock.
I became supremely tired of keeping track of the keys, unlocking the U-lock, finding a suitable sturdy metal bar, and re-locking the u-lock - especially in the winter when the lock was cold and my hands would freeze.
So here's a way to get all the benefits of the appearance of the U-lock, without the hassle.

The idea is similar to this: Homemade U-Lock

Step 1: It's Everywhere, and Free

If you want a broken U-lock, it will appear.
They drape, discarded, over the high-theft-area bike racks.
They fill dumpsters at your local bike repair shop.

I got 22 of them from my local campus police's bike auction.
They cut off the locks and sell the bikes.
The locks were in a big pile, conveniently cut near the joint, and free.
I walked away with an armload.

After that, all you need is superglue, some magnets, and rocking good tunes.
I like "Ghostwriter" by RJD2.
The Smiths are good, too.

Step 2: Assemble

Glue the magnets in, so they don't touch, but pull the lock closed when they're near enough.

Step 3: Golden Locks!

Now you can hook your bike up to those convenient steel tree-replacements that fill your city.

I feel that if anyone was going to go to the effort to steal one of the bikes I ride, they've already got it. This is just to filter those who see "unchained bike" from those who really want it.

Works well enough.