I would get a small SLA battery instead of trying to keep the alternator going while you're stopped.
You can get a 1.3aH 12v battery for $18 at batteries plus. It weighs 1.4lbs.
Werker 12V 1.3AH AGM Battery w/ .187 Terminal - WKA12-1.3F - Batteries Plus
That little alternator looks like it has some potential.
I wonder if you could just yank a 12v regulator off a scooter/moped.