I flew a DC10 once from Los Angeles to Gatwick with a spare engine under the left wing close to the fuselage. It had fibre-glass cowlings built around it to reduce drag.

I remember that it only reduced the cruise speed and increased fuel burn by very small amounts.

The only problem was the time it took to mount and un-mount the contraption.

Back to carting a spare engine -

Here's some upclose shots of a 4th pod on a DC-10-30. (DLH methinks)
(Scroll halfway down the page)

Engine Out Ferry (inoperative engine still fitted) is available as a Flight Manual supplement for most 3 and 4 engined aircraft but not all operators opted for it. It was not part of the basic Flight Manual.
A previous employer of mine experienced 747 pylon damage away from base and Boeing offered to 3 engine ferry (with engine missing from damaged pylon) back to base. We opted to replace the pylon where the incident occured as there were reasonable facilities available. Back in the 60's a colleague's wife told him she saw a DC8 with only 3 engines fitted landing at Long Beach. He scoffed and said maybe only 3 smoke trails ie. one engine inoperative, but next day had to eat humble pie when he found a DC8 had been ferried in with a damaged pylon (no engine fitted) for repair.
Lockheed L1011 3rd pod went under the right wing just to be different. L1011 performance penalties and limitations for 3rd pod use were fairly severe compared to 747.