I’m trying to get the reed in tune and not cut out.
When the bridle is moved upwards it’s in tune but cuts out.
When it’s lower down it’s stronger but flat.
I’ve tried putting a small blob of blue tac on the tongue near the bridle, putting a hair at the bottom of the tongue, and applying a flame to the lower part of the tongue for a fraction of a second.
I’d personally avoid the rubber band; on some drone reeds I’ve had it work, on others it’s been too loose. Instead, what I’ve done is wrap a few wraps of a thin (like sewing) thread around the reed to make a bridle that way. It should be tight enough to keep things snug, but also be able to slide (well, gradually move, anyway) the bridle back and forth.
I’ve seen small black rubber rings being used on composite reeds, so they must work. I usually just use some waxed hemp, tied on tightly but which can still slide up/down.