You can make a heat stick or mount a hot water heating element in the bottom of your boiler. It will need to be the correct watts for the amount you are boiling (ex. 1500watt for 10-15 gallons. You can add two and run both to speed up the initial process then use one to keep a good constant temp. A variac is a good way to control your temp. Ebay has them cheap. It too needs to be rated for your amps. Also running two will require you to plug into two outlets on seperate breakers in your home.

What About A Hot Plate,20 Bucks At Lowes