I have tried a lot of hand warmers, but have settled on the rechargeable electrics for now. They cost some, especially if you get the larger 10000 mAH size which can go all day on low. They are a smooth, egg shape, and when I got them, they would fall out of my pockets when I sat down- The pockets in a Striker Ice jacket are not ideal.
Anyway, the hand warmers have a wrist strap. like comes on lots of hand held electronics, and we started looping that around the elastic tightener on the hem of the jacket. That worked, but since it was outside the jacket, it could hang up on things and pull the warmer out. It kept it from getting lost, but was not ideal.
What we are using now is a loop sewn into the deepest corner of the jacket pocket. You can turn the pocket inside out enough to access the corner, and we made loops out of a 300lb dyneema cord I have, but there are dozens of things to make a loop out of. My wife zig-zagged that onto the pocket lining, and it was done. We looped the wrist strap through that loop, and now it is inside and out of the way. We can remove the warmer for charging, but we usually leave them in all winter and charge them in the pockets. There is enough slack to pull them out to turn them on and off, and for charging access.
Ice fishing coats have a lot of pockets and tabs for all of the stuff we need to keep handy, but don't want to lose down the hole or in the snow. I think this would be a good addition for many people, if someone wants to pass the word to CLAM or Striker!
