not sure about preset heaters. but any adjustable will heat a half gallon pretty accurately. your problem would be whether they fit in the 1/2 gallon.
The hagen elite 25ws on Amazon will do the job. it's really not about wattage. rather I think it's about ... quality and accuracy.
This Jar was is under .5g. I kept a newcomer in there for a few days wholst hunting for a new home. it was tall enough to fit the heater. that's all that mattered