I don't disagree with Mel that by opening up the case and removing the glass we may induce a leak, however since we maintain an experimental aircraft that does not have to meet the requirements of 14 CFR Part 23 we can open the case. If we had to use TSOd parts we would not be able to put foriegn made flight instruments (non-TSOd) in our aircraft which can be purchased through Aircraft Spruce, Wicks, etc.. Where there is an exception is whenever the component transmits on any controlled frequency, such as a transponder or Comm radio. This is why we all built a homebuilt aircraft so if we want to put automotive spark plugs, alternators, or instruments in our project we can.
I was even told by my local FAA FSDO that we do not even maintain our AB aircraft in accordance with 14 CFR Part 43, except for appendix E & F and that is only for accuracy of the pitot static system (app E only if we fly IFR). We maintain them IAW the Operating Limitations that are part of the C of A.