Buy a vacuum pump. It doesn't have to be a monster one. It would not take much to empty such a small cavity of water vapor. The focused can remain fixed, and you don't have to worry about replacing moisture baggies over time. There are many epoxies that could make such a nice seal.
A note on using TECs in a portable host...
well.
They take tons of current in order to function at all -
When they do function they are very inefficient -
They must be actively cooled to keep the cold side "cold", you must use a fan -
They can actually heat your target if your active cooling isn't good enough.
If by "150mw" diode, do you mean one of the 638s? Are you going for cryo cool for the lambda shift? Or just for heatsinking?
After considering these things, do you have a heatsink large enough? Will your cells be able to supply enough current? For how long?
Let me know if you've got ideas - for now these are the only things I can remember to consider.