There is a heater called a fireplug and Hydor makes an external. I don't know if the Hydor's are available in the US though.
Someone on the AB board made an external type heater. I don't remember who. You could search for it in the DIY section. Basically the outflow from his cannister flowed up into some approximately 2" PVC, then to a "Y" type fitting that was turned upside down so that the "leg" of it overflowed into his tank. A screw off cap with a slot for the cord allowed the heater to be placed in this. I believe he added a short section of PVC to the outflow of it to reduce surface turbulence. Make sense? I may go see if I can dig up the article. My explanations suck. lol
It was an ingenious solution though.
Disclaimer: Any errors in spelling, tact, or fact are transmission errors.