Find a cooler that lets you connect it to the engine coolant and trans fluid. The engine will keep the coolant around 170-190 and the cooler/heater will do the same that’s how the factory cooler/heater works also. You can tap the heater core lines easily and run relatively short lines from the trans just mount cooler to frame rail
thats what I did, and it does the job perfectly. the marine oil cooler i linked to in the writeup is very compact and very effective. make him use it!!