Well, after years of planning, my wife and I have just taken the plunge and bought a proprty on acres in Arcadia which is about 50km north of Sydney. Her motivation is a tree change (albeit one still within the greater Sydney area) whilst I only had 1 criteria, somewhere to build an obs. OK, 50km out of Sydney doesn't give me pitch black skies but it's bound to be an improvement on my current location which is close to the geographic heart of Sydney.
I'm currently investigating the idea of modifying a 20' shipping container as seen in the attached sketchups. The reason being that my brother-in-law owns a business that sells & modifies shipping containers. He seems to think such a project should be pretty straight forward.
My main concern is the heat factor in summer. Shipping containers are notorious for getting very hot inside. I'm hoping that this can be overcome by adequate motorised ventilation, raising it ~30cm off the ground and by using a suitable exterior paint.

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I'd be interested to know if anyone has managed to tackle the issue of an overheating obs?
I'm meeting up with my brother in law on Friday to come up with a design so that work can begin. I'd appreciate any comments / suggestions that anyone may have re such a project.
I'll post updates as this progresses.
Cheers, Matt.