Chewing gum in the nose of my P-38 (few sticks of well chewed Juicy Fruit did the trick).
Didn't work on the B-29. Shoulda gone with a whole pack of Big League Chew. Probably gonna take less of a kludged approach to the B-29 actually since it is a cut-away.
Melting fishing line weights into plates then ever so carefully applying in reclusive areas on the model still warm to the touch so as to stick to the interior, but not melt it, was an old trick my father mentioned he used to use.