There is several ways to do this.
One is to design one, these are electronically quite simple, I have several designs here I have made over the years.
These I can post the diagrams if there is any interest, although it will require electronic building skills or have someone make them up for you.
It is possible to adapt a generic Chinese unit, but requires knowledge of the particular AVR you are working with.
Another very simple way is to use a saturateble inductor in series with the main output windings, the secondary winding is placed in series with the brush field supply phased to increase field current under load, this stabilises the voltage but requires a bit of fiddling to get this just right. This method was used in the Markon , early Honda and similar units.
If its lights flicker you are worried about, a mains isolating transformer can be placed in series with the output load, the secondary of the transformer is shorted. This works similarly to the old arc welders with the choked air gap current control, and lessens the tendency for large voltage fluctuations and distortion on the waveform.
Another very simple way to reduce light flicker is to connect a capacitor across the brushes, this effectively averages the exciter current from the bridge rectifier. Its simple to do, perhaps the best component to use is an easily available starting capacitor for an electric motor, These start at around 4 mFd , Id suggest 10 mFd would be close for most alternators.
The downside to this, it will increase voltage overshoot when an inductive load switches off, such as a refrigerator or electric motor.... not an issue if not simultaneously using electronic stuff.
Electronics is the way to go here, Ill post the diagrams if there is any interest. These were designed by me to serve specific units, but are universal enough to be easily adapted to all mains alternators as all designs are electrically pretty much the same..
