Some aux. trans coolers come with a pre-installed fan right on the cooler, as do some Perma-Cool kits, but they are more expensive. At highway speeds, not sure that a fan will do a lot of extra cooling for you, but at lower speeds, would surely help some.
We have gotten some aux ATF coolers from bulkpart dot com, and they carry a wide variety, many of which are Long-made units. Companies such as B&M, Hayden, etc. also sell Long-made units. I can get a nice 12X12"X.75" universal aux cooler/kit for $50 plus tax here at our local AAP and that is what we have often used as well.
A couple of other things to consider, use OD sparingly, such as on easy downhill runs, etc. Better to use the direct drive gear (1:1 input-output speed the same)that may also allow the torque converter to lock-up, which will in turn keep temps lower. Torque converters build a lot of heat when multiplying torque (clutch is unlocked) pulling heavy loads.
Also don't neglect your engine and rear diff. Use lubricants there which can take more heat/load and still protect. Perhaps a bump up in engine oil viscosity and/or use a good syn while towing in hot weather. I would consider at least a temporary switch to HDEO 15w40.
The rear diff could benefit from a heavy duty synthetic 75W-110 or 75W-140 while towing. We use Amsoil Severe Gear mostly and it has been "bulletproof" for us with anything we throw at it. Plus it has great cold flow for the winter.
I'm just sharing what I would do in your situation to avoid problems.
