Good to see that worked for you.
Another method I have used is to use the bolt hole function of the DRO to make another index plate. For a one off job this could be MDF or acylic.
If you were able to silver solder the bar OK then maybe it is not al bronze.
Could it be you are getting the metal too hot so the flux becomes spent. My favoite flux is Bakers fluid.
You could try on another part with the flux you have. Cut off a small bit of solder and place on the joint and...
Love the lamp post engine. A stepper motor makes a good generator and gives a good output at low revs. On this model there is a stepper motor hidden inside the casing. You can get very small ones that could be suitable for this application..