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As others have said, I put my Storm Whistle on a piece of bungee around my corrugated hose. As for Mirror, I used to keep in the waist pocket of my Outlaw with Rogue bands, but now, on BP/W, I just put it in my pocket. (Apex pocket-shorts clone.)
My whistle is attached to my BC inflation hose...mirror? Dont use one.
You say you have a Sherwood , can you use that with a SS backplate, do you know? I've trying to find some info about it but haven't had luck. I would love to use the soft plate with a backplate/wing as I like the integrated weights and front pockets of it.