It's simple. Trace the wires from the write notch sensor switch and wire a toggle switch in parallel. When the toggle switch is closed, it acts as if the write sensor switch is closed. I have done this mod on several Disk II drives. I assume the UniDisk is similar, though maybe not identical. I won't take one of my drives apart and list exactly where to solder, since I don't have the same model drive you are working on. With a three way switch, you can wire it as write-protect always, normal, or write-enable always.