Relative Humidity above 65% will promote microbial growth and there are now several strains of fungi known to degrade polycarbonate, so those old predictions of plastic lasting in landfills for 1000 years are looking very wrong already.
Why reinvent the wheel? The traditional way to store something long-term is to bury it in an ammo can in the backyard because it's both cooler and there's far less temperature fluctuation underground. The silica gel desiccant mentioned in this thread can maintain relative humidity in a sealed container at 25% indefinitely, or you could fill the empty space inside with argon or CO2 welding gas or dry ice before sealing it up if you want 0%. If you can't get an ammo can, a cheap plastic 5-gallon bucket with gasketed lid should work just as well for a time capsule vault. But you'll probably want to bury it in the desert if you want it to last hundreds of years.
I too had CD-Rs regularly fail within a year of music storage in the car, but CD-Rs I burned in the 1990s and kept in a cool basement with no special treatment are still readable today. The initial error rate of higher density DVDs was much higher but still acceptable, but on Blu-Ray was so shockingly high that I never seriously considered it for data storage.