The actuator from Jaycar is 33mm at its widest point so I suspect I will need to sand off about 6mm total. Plugging in to a 12v < 10A source throws the rod hard out or in depending on polarity.
Just sitting down to draw a circuit diagram for the relay to control it via Arduino and I notice this article.
Switching and reversing arduino relay shield which is exactly what is needed. Thanks!
This looks better. Most relay circuits for Arduino seem to have transitors and diodes. This one is designed for reversing a motor. More to look at obviously.