Hi Robert,
i don't know if this has been sorted yet but I could suggest that Marrickville Community Centre (Addison Road). There is a place in the back corner (south west) which is the concrete foundation of a former structure no longer standing. Cars can access this area. It is not secure in the sense of Lock up shed but the Centre itself does have a resident caretaker. Not sure who to speak to but my husband is involved with the Casa Latinamerica there so I can find out more if required. Let me know.
Thanks,
Magda