PETG is also not resistant against toluene.
A month-long contact under ambient conditions results a complete swollen/disintegrated PETG material (PDL rating 0), and an year-long exposure even causes whitening [1, p. 2660]:
Chemical Resistance of Thermoplastics; Woishnis, W. A., Ebnesajjad, S., Eds.; PDL handbook series; William Andrew: ...
You need something flexible and airtight. Could neoprene fabric, like it's used for diving suits do the trick, esp. for prototyping? Otherwise, I thought of injection moulded silicone, as it's used in diving goggles.