Bipolar Transistor PTAT Cell and setup
If you keep the current levels reasonable so that self-heating is not excessive, the temperature should track ambient fairly well, with some offset. A heatsink would reduce the offset. You can predict the offset from the thermal resistance figures found in transistor datasheets. Hint: You probably want to be more like 10uA/100uA than 1A/10A. 

You could use the Widlar current source, but it will have a temperature dependency. You could consider just using resistors in a 10:1 ratio connected to a relatively high voltage (think about first-order correction schemes for Vbe). Or just keep the current sources at a constant temperature. 

Note also that you could use the same transistor for the two voltages, just time-multiplex between two currents and look at the difference in voltages.