From reading on the web, silicone seems to be the best material for gas permeability. I decided to take out the sensor and wait for my silicone membrane to arrive.
While inspecting the sensor, I noticed a nice slimy feel to the membrane. Bio-film had formed. I can see that building up and blocking gas exchange. If you were to design a housing, it's best to keep in mind that you'll need to wipe the membrane clean periodically. A weekly routine should work.