If a Heath Cantenna is overheating all that much that indicates that it is
being used at well beyond its rated power. Added cooling is not necessarily a
solution. The stress on the resistor comes from thermal sources as well as
electrical sources. Even with "adequate" cooling, if you exceed the current
handling capability of the non-inductive resistor it will shorten its life.

An alternative solution is not to use a Cantenna at all. Use an inductive
resistor with an adequate rating and an antenna tuner to tune out the
reactance. Another approach is to use a 40m antenna as a dummy load. Wait
until evening, tune up on top of a foreign broadcast station, and use that for
testing purposes.

73s