What worked the best for me is getting some dirty mice substrate from the pet store, from the mice enclosure... you know, with their scent and poop and everything. Then after the rat is defrosted, roll it around in the dirty mice substrate to fully coat it with mouse scent. And I just keep the bag of the stuff to use this way for many feedings, until the snake is taking rats consistently.
One of mine actually still won't take a rat unless it is coated in mouse scent in this way.