So thought I'd share what I came up with: a $20 bike flag and $20 worth of 15mm pvc pipe and fittings from bunnings. Overall height is about 3.1m, weighs next to nothing, base can be duct taped to the roof rails, and the flag screws off.
Definitely not fit for a Simpson crossing, but quick and cheap solution, and I think it'll stand up to the odd day on the beach. Will test it out this week in SA and update on how it goes...