You can get tiny little bronze or ss clamps for a few pence. They have grooves for 2 bits of rigging wire and a screw in the middle. I have no idea what they are called or what they are really meant for.
Put small loop of bungee in 1 'slot, clamp to backstay using other slot at appropriate height. Then hook the ensign between the bungee and another bit of bugee attached to either another such clamp, or you can often tie a bit of bungee where the backstay bifurcates.
If you have an insulated back stay, use a clamp-on cleat above the insulator to secure the halyard. So, from top, rolling hitch, little block, halyard, flag, cleat, insulator. That way the whole flag set up is "hot".
It would be a bad thing to have the rolling hitch/little block above the insulator, and to secure the halyard to something earthy, like the pushpit. HF set (or at least tuner) would do strange things when the halyard is wet!
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I have used some plastic clips (which do not corrode the stay) that I found on ebay. They are brilliant. Look here
[/ QUOTE ] I know the guy who invented those - we have a couple of pairs and they are amazingly good. Hold all manner of things onto shrouds - clothes lines, radar reflectors, ensign etc.