When I first longarmed I put together "practice pieces" made of the cheapest fabric I could find for top and bottom and a layer of batting. I usually made a piece two yards long and width of fabric. I used this to practice first time machine use and patterns before using on a real quilt. You can use all muslin but sometimes having a patterned fabric lets you practice working around the different motifs.
Later these practice pieces made good dog blankets. I'd cut them in pieces about 22 x 30 and turn over the edges to finish. Nothing fancy at all but my DIL loved them for her dogs. I also made a cover for my longarm from one of my practice pieces.