acindrich,
It's very possible that you don't need to run an
oil cooler at all on your car. My suggestion for using the cardboard, was a
temporary fix/test to see if you could get the
oil up to temp. Thrown a piece of cardboard on the car and see if you notice a change.
During the winter months, I run a thin piece of black rubber mat with some holes drilled in it. You wouldn't even notice it unless I pointed it out. As Bob mentioned, the
oil thermostat may not work if it's extremely cold out.