What we need is:
An FTDI adapter in the shape of a USB thumb drive with 6 holes to in the front end to plug in a flat cable with male "DuPont" pins on one end and female on the other and a 3.3V - 5V - OFF switch so it would work with either MCU voltage or switched off for communication only, and would also have a barrel jack for "external" 5V from USB, with a 450 mA resettable fuse of course. Also a switch from DTR to RTS or whatever.