Safety of lateral (flexible) flags to prevent cars from passing too closely and signaling
Going straight from the right turn lane is a bad idea, and very few legal systems allow it. Partly because it's suicidal, and partly because it's stupid. Either way, the legal authorities have to clean up the mess. So you should, and almost certainly are legally required to, enter the "straight ahead" lane in that situation.

I typed a rough description into the google and got some examples of reflectors on poles like the light you're thinking about.



You could easily modify one of those, or build your own. I believe they're legal, at least in Australia (where I live) but I suspect that no-one would pull you over if it's not (except in NSW, where I live, for reasons to stupid to go in to).

Note that the problem you're trying to overcome is not drivers who are actually blind, but inattentional blindness. The photons are reaching their eyes, but their brain is filtering out all the distracting irrelevancies so they can actually operate their vehicle.At best cyclists fall into the "why would I look for a cyclist, there are no cyclists here" subconscious category of "things not to bother looking for". The way round that is to have so many cyclists on all the roads that motorists know they're always there and consequently look out for them.

Overall, though, it sounds as though you either lack confidence or experience, or you're riding on very busy roads. It might help to browse the safety tag here  for other answers.

(and my browser is not letting me paste any more links in, so I'm going to leave them out. Sorry)