This is the easy part:
Copy the branch of the BGR that you produce the ΔVbe.
This will give you an almost new ctat voltage output.
Apply this to a high gain OPAMP.
Apply the output of the OPAMP to the gate of a MOSFET.
Connect the IN input to the Source of the MOSFET and via a resistor to VEE ( or gnd).
Then mirror the output current.
By trimming the resistors of the copied stage, or the current flowing there you can achieve a highly insensitive Ictat.
The one I made has a good curve around the center temperature and a typical variation of 10nA.
D.