1. The photo is self explanatory.
2. Use a die grinder and carbide burr to do the cutting.
3. Make the square washers from a piece of flat bar (3-4mm thick).
4. Drill the hole in the flat bar washers to 14mm.
5. To make sure that a full nut goes onto the bolt, remove the original captive washer from bolt.
6. Take poison of choice
7. Weld square washer after tightening bolt.

Elongating by bolt diameter (14mm) moves caster two degrees. Four degrees change could be obtained by doing similar elongation to back mounting hole in opposite direction.