You can also buy heat pads fairly cheap that are pre wired and made for reptiles

The method that I have used with lock washers and screws or rivets, I feel, is safe. I would never suggest anything I don't or wouldn't do myself. It is easier then soldering and better then the clips that are sold for it. I would recommend what ever method you choose to use that you insulate any electrical connection with hot glue or a electrical tape that its rated to withstand some heat. I use hot glue on all my connections.

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