I defrost mine in warm water for 20-30 minutes with a warm water top-up 5 minutes before feeding, I then pat them dry with a cloth and introduce... works pretty well as the whole body of the prey is heated through.
I struggled a bit when I was using a heat lamp to warm up the head of a defrosted mouse/rat... Also by using the water I can warm up a heap of mice/rats if needed, useful for when I feed my lot on the same day....
Just don't make the mistake I did, and that is to use boiling temp water from the kettle to heat up the prey, man it was sick.... the belly of the prey swelled up and when I picked them up for feeding they popped.... I almost puked there and then........