Oil Still Too Cool Even With Cover

I find that covering half of the coolant radiator works well to bring the oil temperature up into the green.

So, you cover both the oil cooler and the coolant radiator?

I cover at least 1/2 the oil cooler, and since it is directly in front of the coolant radiator this restricts some air flow thru the radiator, so I don't separately cover any of the radiator. 

My coolant radiator is mounted at the front of the engine and my oil cooler is mounted at the back and below the engine. I cover part of each to get the oil temperature into the green.

Try on of these, oil temp will no longer be an issue, plus much faster oil warm up time.

https://www.aircraftspruce.com/catalog/eppages/xcomthermostat.php