PS: I forgot to mention that any MLAS box you get for your amps will likely be in a similar decomposed state - those boxes aren't getting any younger.
I never thought about this, but the best way to preserve them is painting them with a clear polyurethane varnish. It will stop the decomposition from time and moisture but I really don't know if it will affect the ability of the box to protect the unit from shipping. Given the weight of this stuff, that is a consideration indeed.