Hey folks.
I was looking to get a U-lock just for extra peace of mind, but wasn't sure what would fit other than the Yamaha one. After taking a few measurements, I took a punt on the ABUS Granit X-Plus 54 (30cm version) which gets good reviews.
It fits a treat with millimitres to spare. I got it for �56 from Amazon in the UK. It's surprisingly light too.