Good morning: In Anstistatick foil, at most boxes for 3.5s, labelled, protected from fire and water within the storage temperatures (one usually comes with the other), absolutely safe from mix-ups and safe from theft and perhaps (ironically perhaps) once a year with Restore test (and also that you test if a new interface would become popular instead of SATA, how that would then affect your plan).

Business-wise, I had many customers who simply wanted the year-end backup to HD once a year. A few then also saw the point in a restor test and "let's see if we can still read this).
Some used a bank safe deposit box, others at home. Well, the safe deposit box would be safe.
Most of them could still be read out 5 years later without any problems, in one case the customer or a sepperl took the discs out of the "archive" prematurely and used them... then...

Have fun...

Many important points have already been mentioned. I would like to add two points specifically about storage: even in a secure safe deposit box, there is no guarantee that one of the neighbours will not keep a strong magnet. (Your bank would have to know how far the safe deposit box itself is protected against fire and water, but against burglary and vandalism the protection is logically the best). Then there is the "Magnetic Field Breakdown". Most sources speak of a loss of 1% per year when not in use. So if this disk sits unused in a locker for, say, 5 years, even under the best possible environmental conditions, about 5% of the sectors will be broken. If data was stored exactly on these sectors, it is gone. Therefore, a hard disc is not the right medium for long-term archiving (10 years, 20 years, "forever").