Just get some THG heat tape. Rather inexpensive and very easy to wire. They have plenty of youtube videos showing how it is done. If you don't want to do that reptile basics will do it for you.

http://www.reptilebasics.com/heat-tape/6-heat-tape/

"We can also assemble your order to your specifications. If you would like us to connect you Wire/Clipset for you please also select the heat tape connection option on the wire set and leave us instructions in the comments box when checking out." per reptile basics site.