Any good hydraulic hose shop can help you. Ideally, for flexible plumbing, IMO, you should use swaged fittings on the cooler if you want a reliable connection. If it's hard connected, then flare nuts are the way to go.
When you say the ATF is hot, . . . . how hot? It's normal for the ATF to heat up (quite a bit) under load. The engineers that initially sized the ATF cooler designed the system to keep the ATF within acceptable limits at maximum towing capacity and extreme temperatures. Generally, they don't guess, especially on truck chassis. Are you having a specific problem, or are you thinking that you might? The numbers tell the truth.
Chum lee