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Without the access to hardware stores as you have in California, the fastest for me was to order brackets like these:I would have just threaded the 4 inch pole and used a 3" to 4" bell reducer and an appropriate length 3 inch pipe nipple, (all galvanized) all this can be had at most hardware/plumbing stores.(and be sure to use some pipe joint compound to keep things water tight)
Any plumber could do the threadding job.
Unless your into doing the job yourself and have the time.
The specific ones I have fit 89mm (3.5 inch) poles and down to around 40mm. And less than 10 GBP...
I am sure you can get larger sizes as well.
A bit tricky to adjust, but once you get the hang of it, you can pitch the attached pole in very fine increments to ensure absolute verticality...