The larger the load the more waste heat, that’s absolutely normal that it needs much more cooling than is available. If you got the same performance from some other less efficient system (Jetsons are quite efficient), then the problem would be much more severe. You definitely need more cooling.
There are a few possibilities, more than I will list (and I don’t know of a specific IP65 chassis). These are just some starting ideas. FYI, some require removing the heat sink and replacing it, and I’m not so sure that is itself a good idea. The actual heat sink attachment might change depending on whether this is a developer’s kit or if this is a separately sold module sold with some third party carrier board (you might want to mention if this is a developer’s kit or not).
Do you have sufficient power available to use a Peltier cell? They directly transfer heat from one surface to the other surface, but at a cost of requiring electric power greater than what they are cooling. Just guessing, you’d probably be using another 75 watts or more. The heat sink of the Jetson would have to be removed, and then mounted to a Peltier cell; the hot side of the Peltier cell would need a cooling surface exposed to the outside world, e.g., if your case is aluminum, then the Peltier hot side would go to that aluminum chassis, and the cold side mounted to the Jetson in place of the heat sink.
If you have an unpainted aluminum chassis, then you could remove the heat sink and use the chassis itself as a heat sink by mounting the Jetson to that in place of its original heat sink. Heat sink mounts do need to be highly “flat” to be efficient, and then use something like heat sink compound or a thermal pad as well.
If the chassis itself is completely waterproof, and is a good conductor of heat inside and out (e.g., not painted, and not plastic, but instead something like aluminum), then there is a possibility of immersion liquid cooling. The fan would be removed, the Jetson mounted in the middle of the chassis inside, and then filled with a non-conductive dielectric designed for that purpose; you might or might not need to protect some of the Jetson with conformal coating (most components would simply be immersed). The dielectric liquid can be expensive, add weight, and you’d need to know the chassis is perfectly sealed, but this would be rather efficient.
If you know the device is not going to be underwater, but say suspended such that no flood water or other water would reach it from the bottom side, and that something like rain could hit the top, then you could simply leave the bottom open and put a simple fan in (the default fan of a developer’s kit might be enough if the air itself is not badly trapped, but temperature would go up versus open air).
You could reduce heat generation by using lower power models, but you’d lose a lot of performance by the time you get to where the Jetson could operate continuously in a sealed chassis. This wouldn’t be IP65, but maybe you don’t need IP65’s full protection.