After a recent camping trip on the outskirts of Kilkenny, Ireland and a night of freezing rain and driving wind I was saved by a trip to a Guineys (If you know, you know).
I picked up a fleece-covered hot water bottle for €5.

Second night my feet were toasty and I was able to sleep through the night (Well one more fill up at 3am at the open kitchen )
Once emptied the hot water bottle went into the right hand pannier along with my sleeping bag and inner liner with no discernible difference in closing the pannier.
Recommended piece of kit if a little old-school. (Or maybe I am getting older and feeling the cold more).
I picked up a fleece-covered hot water bottle for €5.

Second night my feet were toasty and I was able to sleep through the night (Well one more fill up at 3am at the open kitchen )
Once emptied the hot water bottle went into the right hand pannier along with my sleeping bag and inner liner with no discernible difference in closing the pannier.
Recommended piece of kit if a little old-school. (Or maybe I am getting older and feeling the cold more).