[RGene7001]

I use nothing.
Transmission temperature is monitored by vehicle computer unless this is a very old model. In the worse car scenario there should be warning on the screen or limp mode engaged.
Usually there is not much of heat production when torque converter is locked and it stays this way in all times except the moment of gear change or while standing in D or R.
Once I touched my cooling lines after driving about 500 miles over some rolling hills and the incoming was only moderately hot while outgoing ( to the transmission) very close to ambient temperature.
I usually lock out 7 or 6 and 7 gear depending on wind and terrain.