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.
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Reason is a scoundrel, stupidity is direct and honest. –Ivan Fyodorovich Karamazov
Man's mind, once stretched by a new idea, never regains its original dimensions. –Oliver Wendell Holmes