I've tried that but it rattles in mine so I put it in the pannier (which I suspect helps to keep the drink warmer too). Might work with a sock around it as there is a little space?@User - does the mug fit into a bike's bottle-holder, so it can be used during winter rides ?
Received mine today, chose medium and they are a good fit. I'm 5' 9" with a 32 waist.