The efficiency of a turbofan (which is what those "jets" really are) is limited by how big you can make the fan part. This is a basic property, and applies to propellers too.
If you look on a modern airliner, they have "high bypass" turbofans, meaning the fan part is a lot bigger than the jet part. They've basically made the engines as big as they possibly can: most of them are mounted in front of the wings, not slung under them, so the intake and fan can actually extend up above the wing, and often the bottom of the cowling is flattened to give a bit extra ground clearance. Contrast that with older jets that had these long skinny little engine nacelles slung under the wing.
Your ideal design for an efficient jet would be one gigantic engine right in the middle, but unfortunately that's where you want to put the passengers. Your next best bet is to have two engines that are as big as you can get, to either side. A twinjet.