A conversion cap for using a pair of CR123 batteries
to power a SDU-5/E Distress Marker Strobe light can be found here

http://www.prc68.com/P/Prod.html#B

I have no experience with these - just Googled for it because the
question came up above.
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)