The Computer Method

First, put the frozen rodent in an airtight plastic bag (when I buy frozen mice from the pet store, they give it to me in a fish bag).

Then, put a weight on top of the bag and have it hang off over the back of your computer, with the rodent right located where the exhaust air comes out. Make sure there is some space between the bag and the computer, so you don't block the ventilation.

This is twice as fast as laying a rodent on a spare heatpad (2-3 hours), though I haven't tried other methods.