I am an idiot

+1 That's the answer right there. They are also sometimes (confusingly) called inverse shims.

Inverse shims are campy whereas negative shims are NJS.

Did exactly the same when I first got my IRO start of last summer. Negative shim from BLB £1.50 will see you fine. Tell Jan in there Ben sent you and he'll probably let you have a used one for free.

I don't want to seem unappreciative, but how can you get a negative shim? The diameter of the clamping area on my Mono Lever can only clamp around a smaller diameter than my bars. A shim can't reduce the diameter of my bars... Any chance someone has a photo of said shim, or a link to one?

Seems a little bit like you want to make me look like more of an idiot...