The question was: How do you build downfield routes that stretch defenses horizontally (from sideline to sideline)?
An example of a short stretch is all curl. There you have 3 "short" receivers (tight end over the middle, backs in the flat) and two curling back receivers (outside guys) who come under the deep shell of the secondary and stretch four underneath defenders with five guys. Sid Gillman invented the play and Bill Walsh ran it for years and years.
See this article for further discussion on concepts and horizontal and vertical stretches.
Here are some thoughts on applying this procedure to routes farther downfield:
Examples of "downfield" routes that still use horizontal stretch concepts are the three-verticals (corners and a post) which is used to "horizontally stretch" two deep safeties. Also the four verticals play is a deep horizontal stretch, where you use four receivers to stretch three (or, more simply, the two inside receivers to stretch the middle safety).
I'm not sure it counts as sufficiently "downfield" but other common ones would just be a 10-12 yard out by #1 with a curl or seam by #2. You read this out to in. Often the RB sits over the ball so you get a kind of 1-2-3 horizontal stretch.
Note that several of these routes (like the three verticals with the corners and the post in the middle) employ both the horizontal stretch and the vertical stretch. For example on three verticals you stretch the two deep safeties horizontally with the two corners and the post, but you also stretch the cornerbacks/flat defender hi/lo because you send the runningbacks or a TE type player to the flats.
A final thought on this question, however.
This may seem like a simple question, but it really gets to the heart of how good passing concepts are built.
They are built with sound stretches, often layered over each other to put the maximum pressure on the defense. They are finished by making each route good versus man to man, or including a man to man concept.
You cannot build pass concepts that beat all potential coverages, but your goal is to make the defense work at stopping you and make them pay for their mistakes.