Might I suggest using an RC servo instead? I don't know the level of experience you have with the Arduino, but for those who may not know, servo control is very straightforward.
I'm working towards a very similar grill block, but it looks as though you are closer than me to making it a reality.
I have an on-board Arduino performing other duties, and have CAN communication working in testing. I've envisioned a similar shutter, but with aluminum slats, the Arduino polling the ECU for coolant temp, and using a PID algorithm controlling the slats via an RC servo to the minimum required opening.
I'm currently using one of these servos:
125G EXI Servo B1228
with one of these for 12V to 6V conversion:
Exceed RC UBEC-3A (2-6S Lipo Input)