Good tip on the pre heating👍 We’ve got an insulated jug filter coffee machine and do the same.I bought a 350ml metal thermos last year that fits in a bidon holder. (Great idea about the sock to prevent rattle @Oldhippy - I'll try that instead of a piece of pipe lagging foam). View attachment 618511
It works great. If you need it to stay warm for a VERY long time (>2-3 hrs), preheat with boiling water before pouring in your drink. Job's a goodun!