I like to print ABS because the prints are pretty tough and can take a lot of abuse, but my printer is pretty big, and though it is fully enclosed and insulated, the bed heater alone is not enough to raise the enclosure temperature to 50C for printing ABS. I recently added a heater to bring the enclosure up to about 50C for reliable ABS printing.

A salvaged 500W heater taken from an old Stratasys printer (ironic, isn't it?) provides the heat, while air is circulated by a 24V fan. Power is switched by an industrial surplus SSR (125A @ 660VAC!), and enclosure temperature is monitored by a generic 100k thermistor. A PID algorithm running on the SmoothieBoard controller regulates the enclosure temperature.