Rather than let it die (slowly) in the freezer, you could just kill the rat instantaneously and then toss it in the freezer if that's what you choose to do. Either place it in a bag and give it a good whack on something solid, or place a ruler (or other flat object) behind the rats head, and with the rat lying on a table or something, pull up sharply on the tail (breaking the rats neck/spinal cord).