It sounded like the poster wanted to make them from scratch. You know, engineer the system and determine how much iron to cut, figure out how many wraps to put around the iron, etc, etc.
I agree cccapt you can buy and assemble the pieces much cheaper.
Here is a site for you pc
to at least get an idea about what you need.
If I'm not mistaken (
moderators may disagree) the electronic type will save you on your electric bill and more importantly heat in the fish room.