The Gemara (Shabbat 49a) says that tefillin must only be worn with a "guf naki" (clean body) which is defined by Abaye and Rava respectively as flatulence and sleeping. Other Rishonim, like the Maharam (Orchot Chayyim I Tefillin 3), and Achronim like Arukh Hashulchan (OC 38:6), and Chatam Sofer (Shabbat 49a) likewise include improper thoughts as a disqualification.
My question is, does anyone know of any responsa, psak, or halakha (with preference for more modern ones) that addresses situations that clarify the requirement of "guf naki" for men? Does anyone clarify if it is dirt, blood, or lack of bathing (weekly? monthy? less than standard?) that would qualify as non-"naki?" For example, does any posek deal with wearing tefillin on a camping trip when showering is infrequent and good hygiene is difficult? Or about wearing tefillin after a red-eye flight or all-nighter?