Are you using the lightweight version of a Hydro Flask? If so, does it keep your drink hot a reasonable amount of time?

Not sure you'll get much lighter for a durable metal bottle, but if you have one of those old OR bottle insulators for a Nalgene, maybe do some experiments at home to see how well long they'll keep your drink hot. I know Nalgenes are supposed to handle boiling water. I wouldn't try it with a Smart bottle or similar.