You'll be fine. Clamping force is massive with most clipon arrangements. Adding another clamped interface shouldn't be an issue. Make sure that whatever you use is polished very smooth, is clean, malleable and not kinked when you wrap it around the fork tube. Aluminum sheet is a great option. Don't worry about getting a perfect fit either. In fact, for aesthetics, you might want to trim the shim a little thinner than the clamp surface of the clipon. The frictional resistance will be the same.
A polished surface will result in much better and more consistent friction than a rough surface.
