hole fill/plug material

I'm looking for a material (preferably polymeric) to plug a hole through some 0.40" thick aluminum.
Hole diameter: 5/8"
The material needs to have a low MVTR (on the order of ~3.0 g/24hr/m^2/ atm or lower), and not release volatiles when heated up to around 90 C. I've searched on (http://www.matweb.com ) with some luck, so far I've found HDPE, and Ethylene Vinyl Acetate.
Does anyone have an idea of other materials I could look into?
Thanks for any help you can provide.
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mr snipped-for-privacy@hotmail.com wrote:

Cork has excellent temperature resistance, and it will hold itself in the hole if compressed during insertion. It is very resistant to compression set, unlike any solid elastomer. Artificial materials used as replacements for corks in wine bottles are also possibilities. You can be sure these materials are relatively clean, otherwise they wouldn't be used for wine. If you only need one, you could carve it from a wine bottle cork. Those are greater than 5/8 inch. Buying a bottle of wine would be the cheapest and easiest way to get a piece, and you could always send the wine to hazmat disposal (if you're Mormons or something).
A platinum-catalyzed addition cure silicone will not outgas as part of its curing mechanism, but there will be low-molecular weight silicone oligomers which are emitted. That's a big problem for silicone in sensitive applications. You can get special silicones stripped of these oligomers, such as Dow Corning 93-500, but that's expensive stuff.
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