Bill---I have a safe and a safety deposit box but do not use either for cards. Because I collect sets, and lots of them, and display them all in binders, it is not a viable option for me. Insurance and a lot of pictures is the route I have gone.
But I think you should get a cheap slabbed card, put it in you oven at 250 and then up it by 50 degrees ay intervals until you find it's meting point. And then report back. To be thorough you may need to get a cheap slab from each major grading company and report which one holds out the longest
