I had a 92 Dodge Cummins Diesel PU and there was a factory tranny cooler I installed on that since the Cummins put out way more power then the tranny could take and it would overheat. It was frame mounted with a fan that came on at a preset temp. The noise from the fan let you know you were hot and it worked well. There might still be some out there you could pick up cheap and adapt.

1997 T-100 4X4 Xcab,Warn Hubs
02 Camry LE
2008 Yaris