A reptile mat?

Your Nep can withstand temps higher than 80 so cooling is not needed but 55 is too low for optimum growth. 75 to 95 is the best range for most lowland Neps. A submersible aquarium heater with a minimum of 2 inches of water in the aquarium works great, not only for temp. but humidity also. Put your plant on an upside down pot or something to keep it above the water. As Jim mentioned, a reptile mat will also work.