2. A piece of string ?
This is a common accessory supplied with cheap Leisure boule. It normally comes in the form of a length of string attached to two pointers of plastic. Nice try but don't bother. String has a tendency to stretch when measuring opponents boule.
3. Telescopic pen measure.
Ideal for quick check and in the hands of experts can be used very effectively. Limitations are under 12 cm and over 40 cm for obvious reasons. Can be marked to show minimum circle diameter but is a comparison tool rather than a measure.