Inert gas?
The reason for two lenses is because an inert gas is sealed between them, to keep them from fogging. By driling the hole, you lost that gas. Although, seeing the moisture to begin with shows the seal was already lost.
The only solution I can think of would be to remove the gage, reseal the outer edges with a clear epoxy and where you have that hole now, use an inert gas, like argon, to inject it into the hole for a few seconds and then quickly reseal before atmospheric are gets back inside.
The moisture (fogging) is caused from condensation from the ambient air. Get the ambient air out from between the two glasses and the fogging problem should stop.
If you remove that outer glass, your opening the gage to the elements and will likely be ruined in a season or two.:cheers: