Try 7/16
Got something against sae?
If can't find 11mm (and it's not going in a threaded hole) 7/16 would be the closest fit.
7/16 is slightly larger then then 11 mm which isn't a big deal when it comes to the frame mounts, those can just be drilled out. Depending on what kind of sleeve he has for the ca's he might have an issue there.