Current carrying cable/busbar cooling ways in an aircraft
Mentions of cable related cooling at altitude are hard to find, alas.
However , the papers cited below relate to cooling at altitude and deal with the related issues in general.

Suggestions:


Derate
Fan or forced air flow (even though you would prefer not to).
Increase thermally conductive paths - eg  


Provide a metal frame or shell that adds thermal conductivity or radiating area.    
Bond to metal surfaces with metallic "cable ties".



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Garglabet image search - cable cooling at high altitude



This paper relates to electronics rather than cables but looks useful. Note that they suggest that fan cooling is the easiest approach.

Cooling Electronics at High Altitudes Made Easy



High-Altitude Considerations for Electrical Power Systems and Components

Cable not mentioned but related factors discussed.

Slideshow:

Paper



How does altitude affect AC-DC power supplies?



Altitude and Electrical System Design