A length of metal pipe should do it, such as half of one of those telescopic washing line props, knock it well in and lash the pole to it with a couple of zip ties
We went through a number of versions of reebar but found that the flagpoles tended to be too rigid and snapped.
We now use a fishing rod stand from Decathlon, it spikes in the ground and we cable tie that to the trailer A bar, the flag pole then moves slightly and so far we haven't broken a pole since.
I have a 8m fibre rod that I use as a flag pole. I have a large marquee peg that fits snug to sit the pole on and then I have a cable tie set to the diameter of the second section about 3 feet off the ground with three guy lines held down with Deltas. Works a treat and have inadvertantly left it up in wind conditions when only the top section broke but the pole stood fast.
------------- Nigel
March 2012 - Dove Meadows
6th July Moving to Hayle