Welcome to the Quilting Board!
You can use lots of fabrics on the back or even make it reversible.
Whenever we come up short it is a "design opportunity"! Do you have any extra blocks from the front of the quilt? Often quilters will incorporate the "left overs" into the backing. Also, piece a coordinating fabric right down the middle of the backing, everyone will talk about how clever you are and nobody will know you goofed on the backing...afterall, it was a design opportunity. Some of my mistakes and led to some of my best designs! That is what makes is so much fun, to me, anyway.
i do it alllll the time! My measuring tape is broken- can't take an accurate measurement to save it's life! I have rulers that do the same thing- terrible I tell you LOL! If I knew how to post photos I could show you last weeks quilt in fact ALL my backs are pieced (bad tape mearsure bad tape measure!).
From the heart
Quilter's piece their backs all the time. Think outside the box on this one. Maybe you can cut the fabric and put several other pieces in it, vertically or horizontally. It will be beautiful!
Oh boy do I know how you feel Sletinski!? The same thing happened to me. I made a polka dot basket quilt with a white on white polka dot background. Just as I was starting this project all the fabric manufacturers of the world decided to stop making white on white dots. I do not have enough for the back! I have a bit of the border I am going to piece in and pray to the quilting gods that it is enough to make a back!
Had the same thing happen to me on my first bed quilt. I had two pieces of fabric in same open plaid, but different color. Used the cream background on lower part of quilt and the burgundy background on top edge, about the width of pillow and looks like I meant to do that. I piece backs all the time now. Easy, peasy and looks just fine.
I always put left overs from the front on the back. Most of my quilts are on the couch or chair and you see the back. I think of the back as another front.