2-Sided QSLs
So far what you've seen here are all one-sided QSLs, formatted to 4" x 6" size, with all the information on the front.
A more traditional QSL size is 3.5" x 5.5", with both sides of the card used.
Following is an example of a homebrewed 2-sided QSL, measuring 3.5" x 5.5". It prints on a single sheet of paper, which is folded, secured with double-stick tape, and trimmed with scissors: