Convex mirrors are available that will fit the type of arm that goes on the hinges Robin has made/converted for you. The problem is it is the arm, not the mirror, which needs to fold if you have a narrow garage.
The hinge mounts you will have got from Robin use a bolt through support for the arm and if you adjust the tension of the lock-nuts so they can be folded back with a push, they will probably fold back due to wind pressure as you are driving !
Experimentation will be the key. You might find a tension of the lock-nuts that will let you fold them with a push, but will hold against wind pressure when driving. After a while, you will probably need to readjust the tension again as the arms will start to fold in once more under wind pressure.
There are brackets available which fit under the S2 hinges and take the later S3 arms. The S3 arms have a spring-mount base and will do what you are looking for. IIRC the brackets were a L/R original part and pattern copies are available: -
auction: #325210848924HTH
Dormy