I had added the borders on a whim a few months ago and last week I found a curtain panel in a thrift shop that exactly matched my sofa slip cover that I thought would make a good back for them.
I used the finished edges of the curtain (coughbecauseI'mlazycough) to save time and made an envelope style back. Some may say that they are passe' for now because Christmas is over but I say "nope". Since Mother Nature just dropped 10" of snow on my house, which usually gets less than a few per winter, I say snowmen can stay on the sofa as long as they want. Well, they can stay out. They don't stay on the sofa, exactly, because as soon as I place them in the corners, AbbyBeagle comes along and flips them off. Apparently she can't get her hiney into the corners as she likes when they're there. She'll have to get over it. I like my snowmen!