Make sure it's a 125 degree fire safe. And I'd trust tape over flash, even brand new flash, if it's going to be offline for years.
And since you need a second copy to be even close to safe, I'd try to prepay a historically reliable web host for many years of service. And/or get a safe deposit box.
You should know that safe deposits are no longer put into newly built banks anymore and aren't a profit center, so banks are closing their boxes and shipping contents to self storage places, often with items missing in the shuffle. Wells Fargo seems to be the worst for theft. But the bank can and will open your safe and move it to self storage if it saves them a cent.
And since you need a second copy to be even close to safe, I'd try to prepay a historically reliable web host for many years of service. And/or get a safe deposit box.