Start with your 12″ quilt block, and fabric. Cut fabric into 9 12.5″ squares.
Fuse 4 fabric squares and the quilt block onto interfacing.
Cut squares out of interfacing. Stitch 5 squares together to make a + shape. The quilt block should not be in the center, it should be one of the outer pieces, with the bottom of the quilt block being stitched to the center.
Stitch up all four sides to make a box shape. Leave a 5″ gap in the stitching on one side for turning later.
Use the 5 remaining pieces that do not have interfacing to make a second box shape. Place inside the first, right sides together. Pin all the way around the top.
Stitch around the top of the box. Turn the box right-side-out through the hold in the side.
Pin the top of the box, and top stitch around the edge to secure.
Fold each corner, then stitch down about 2″, about 1/4″ from the edge. This will keep the corners looking sharp instead of floppy.
Hand-stitch the hole closed, and your box is done!