You can certainly find miniature heaters that fit under the IC tested. I used another approach, low-cost and very efficient.
Locate your IC on board inside of a small enclosure. If the temperature would not exceed ~120 deg.C, it can be a plastic box, too. Install close to the tested device a high-power resistor, best in a ceramic block. Choose a value suitable to dissipate one to five watts from a variable DC power supply. On the IC glue a temperature sensor. It can be a k-thermocouple or another similar device. I used a small calibrated thermistor (directly connected to a display showing temperature.
Test the device without the tested IC. Plug all openings to avoid air draft. A suitable temperature growth was in my case ~ 0-40 deg.C per hour, with a top temperature mostly 70 deg.C, in some cases up to 120 deg.C (for this I used a 25W auto lamp). For the best results I included a small DC fan to circulate air evenly.
Next to the professional temperature chamber this approach was tested dozens of times, with excellent results. I used the same way to test systems installed in NEMA boxes where external temperature may vary from -30 to +70 deg.C. Very useful with microwave blocks which are not allowed to get wet, always heated over dew point to remove moisture.