I use a fanny pack for this purpose. It doesn't have to be big, just long(ish). Put it in in two pieces, and just leave one side of the shackle hanging out. The bent part should still fit inside, just one straight end poking out parallel to the zip. Pull the zip tight on it. Once you're wearing the fanny pack (turned to your back, obviously) it can't flop down and fall out.
I can easily stuff a 23 cm Abus Sinus, my keys and wallet into a classic-sized smallish fanny pack. If need be, it also takes my "bike survival kit", complete with spare tube, repair kit, multitool, 15 mm wrench blinky and rubber gloves. But that's already pushing it

Also, dutret seems to have reading comprehension issues...