coupling heat to a metal case

[hal feinstein]

Here's another elusive construction question:

I have a 500mw  amplifier module for 2.5Ghz that can get a bit hot.   
I'd like
to couple the heat to the case that houses it which is a metal ammo box.
I plan to strap the module tight against the case but I remember the  
"pros"
often put some kind of  gump that facilitates heat transfer in  
situations like this.
Any idea what the gump is or better yet, can you recommend a cleaner  
coupling method?

--hal f.