Modern aerodynamic designs have mechanical flaps to allow in just the necessary airflow for however much cooling is required. Hot day going up a hill - the flaps open. Cold day lightly loaded going downhill - the flaps close to reduce drag.

An inflatable cone could use the same type of mechanism

Hydrogen especially has this problem because the produces a LOT more waste heat than a pure battery-electric powertrain. One person familiar with the matter once told me the cooling system (fan, coolant pump, etc) on the heat exchanger of a fuel cell hydrogen semi truck prototype used about as much power as a Nissan Leaf.