We handle a lot of data entry at XPO. One of the issues is the users want to quickly switch currencies while they put in a quote or an offer.
Quickly sketched up this UI. Thought could be interesting and rapid.
Of course this UI is gonna be cumbersome if you actually have to select from 20 different currencies. In our case, in a foreseeable future we'd be handling 3 different currencies at most. So I think this would work. Kinda like it :)