cheapest way to heated towels

Luna has spent pretty much the entire day sleeping in the linen closet.  She loves it in there because it's a sliding door with no bottom track so she can swing the door a bit and get in and out by herself.  Plus, it's dark and warm, perfect for naps.  Although she has been in there more than usual since the puppy's been here...  So likes to sleep on the bath towels the most, so they are always nice and warm now.  Good things cats are clean.