Maximum per money order is $1000. Couple that with $1.75 per money order, the rental of the box, and needing extra insurance to get the same protection as in a bank account and it might very well be cheaper to accept negative interest in many cases.
They exist many places outside the US too. It's closer to a bankers draft, which is pretty much like a cheque except you buy it and it's drawn on the issuing organisation rather than your account, which both makes it less risky for a recipient and is what would make it potentially work here (since with a cheque the money would still be in your account).
The biggest issue with this is that it usually costs money, there are often limits ($1000 for US Postal Service money orders), you'll have to expect reporting requirements above certain limits, and you'll still need to buy extra insurance if above relatively low limits if you want your money to be remotely as secure as in a bank account.
The upshot is that while this gets you around bank limitations on storing actual currency, it takes effort to store lots of money cheaply.
