The reusable packs that I have are similar to the Toasterz Reusable Heater Pack sold at MEC and can be boiled to recharge. I have been using mine for several years and they get very hot but only last about one hour.

I also carry the HEAT disposable hand warmers that MEC sells which last longer than the reusable ones.

You can also add a bit of gaitoraid to your water to lower the freezing temperature which will help a bit and adds some flavour.