[pjc50]

Keep the funds in what?

There's about $2trn in banknotes https://www.uscurrency.gov/life-cycle/data/circulation and from my earlier reading of the FDIC statements there's about $24trn in total US bank deposits. So if you want to have everybody keeping their own individually serialled banknotes that's a non-starter.

You can keep it at the central bank, whether that's "postal banking" or "the deposit window" or "a CBDC", but that seems to be politically unpopular for incomprehensible reasons.