If you are going to attempt to "build up" a special bolt, make sure you start with a grade 8 bolt or better. Rember, this is the BRAKE you are working with. A lessor grade may not be strong enough.
As for building the rest of the bolt, stacking washers works well. Other materials to consider are, a short piece of steel brake line, a split roll pin, nylon bushing, oilite bushing, or a brass shaft from an electric lamp.
If you have an industrial fastener company like Fastenal near you, ask to see their shoulder bolts. They may have something close to what you need.
Long live MacGyver!
