There are a few things that I might have done - but I agree with above mostly, in that you 'judge' what is right at the time ... but :
1) Hit the hazards light AND put your left hand open and flat out (like a policeman who may stop you) up near your rear view mirror. Then flash brake lights as they near. This communicates - there is a problem it
may be me or something else. I would have stopped toward the road centre, and further back from the problem.
2) flagging down as Roger describes above, but I might have used a quick flag down signal then the same stop (but with my right arm)
3) I find that holding your foot on the brake pedal, until something comes into view is good THEN you repeatedly flash the brake light by dabbbing your foot, pause on the brake - as if to say LOOK (flashing brake lights get attention), then pause holding them ON, says (look brake), I have stopped for a reason .... and repeat and keep repeating until you see them react. It also tell them that you have stopped temporarily - but you are 'in flow' and 'driving'.
4) if you are not on a very long road, or the car behind, you haven't noticed until very late - hit your RIGHT indicator as if you are going to pull out into their path is they 'overtake' you. That gets their attention, BUT it has the disadvantage that they are thinking about YOU, and not really ahead, and that is a problem, you are relying on them, focusing back on the road, to then see the car ahead and asses the hazard and slow/stop. If they truly think that you are pulling out they may panic but most what I have observed - it makes people 'pause'.
5) He clearly failed to ascertain 'what' you were doing, and failed to see any indication from you or any 'clue'... hence the possible 'power-off' as a show of disapproval, as he failed to assess the potential danger, from your position alone. Not that what you did was in any way wrong.
6) other possibles that I can think of (but are less likely), as I get such a good response from the flash the brake 'message', are
reversing lights and SMALL car manoeuvring. Even 'looking like' you have had to reverse, and then brake, tell another driver that 'something odd' is going on, and causes them to slow. The further indication from you to say 'stop' will help.
even going forwards a tiny bit and right indicator - with a possible steering in towards the centre of the road a bit, (as in s/he can see you moving - try to asses and slow down),
- then you brake and in that time they can start to asses - passing you and then see the next and real danger.
7) Always have your escape route planned too, even forwards - so perhaps reversing back 5 or 6 car lengths if that makes it just that bit safer and for them too, as after they pass you - they have more time to reassess the hazard too.
NB: Most importantly - make sure that what ever you choose to do, is
really really clear. Any confusing by the other car driver may have them state later if an accident were to occur, e.g. I thought the 'slow down' flag was a 'wave to go past the car', and I was so busy with that distraction, that I failed to notice the L car in my path and hit it.
These things are always a fine tune judgement call, but having a small list available to you, is always extremely useful - just like when the motorway is slowing down too, the hazard and brake technique works extremely well, along with escape routes and leaving excellent gaps in front too.