Have an Older Nightrider sport light , 5 D cells battery pack has a O ring sealed cap.
the power supply plug in the side of the lamp head also has a good seal,

the same kit is sold as a Diving light, I cannot Imagine how waterproof the poster needs,
Amateur SCUBA depths is probably adequate.

recently got a NiMH battery pack, to charge it up without taking the batteries out ,
and putting the 5 of them in it in an older NiCad rechargeable battery charger.

maybe the planet bike 2watt LED will be OK , the 1 watt is be seen adaquate
for a short Rural route I have,
and Its been OK for an hour combined, I think over the past 2 weeks,
2AA, NIMH, batteries .
2 spare alkaline in your pocket as a backup
is a reasonable thing to do.