I located some left-hand threaded rod, M5x0.8 thread size, at Grainger:
https://www.grainger.com/category/fasteners/threaded-rods-studs/fully-threaded-rods-studs?searchQuery=Left-hand+threaded+Rod%3A+M5x0.8+Thread+Size&sst=4&searchBar=true&tier=Tier+4
One problem: They're closed on Saturday and late afternoon Portland traffic is suicidal, so it will have to wait until Monday. In the meantime I'll try to figure out how to open the convertible top manually so I can put the car into service mode.