Brake lever clamp too loose for handlebars
Normal aluminum cans are basically .1mm thick, so a wrap gives you .2mm. Most of the smaller scale clamp diameter discrepancies, say from a tolerance problem or from a mismatch like a 26.0 aux lever on a 25.4 clamp area, one encounters clamping controls and acessories on to bars can be resolved with aluminum can shims, or stacks of them. Going up to putting a 23.8 aux lever on a 22.2 section with them gets dicier; other kinds of shim stock will work better, but even there putting 8 layers of can on is probably fine. As long as your shim material is metal and as long as the finished product doesn't slip readily, it's probably fine. Fiber grip/carbon prep can lend a lot of solidity to such hacks.

Use scissors and math to prepare the shims. Don't cut yourself.