i've pieced many many backs===sometimes i just put together large blocks of fabric until it is the correct size - never had anyone not think it was pretty neat (even if it was - just left overs)
sometimes piece large pieces that did not even come from that quilt- so it's a totally different quilt on the back-
sometimes i just start with a large piece and start putting borders around it until it's the correct size-
anyway you choose to do it will work just fine- one thing to consider though- if your top has alot of seams- you may way to use as large as possible pieces on the back so you are not (fighting double seams-top & bottom) when quilting