Another straight seamer here! The other thing I like to do to stretch the fabric when I don't have quite enough, as others have said, is to insert blocks into it. My favorite way is to put extras in the corners diagonally opposite from one another: top left and bottom right I might do cornerstones and then add 2 more of that same block to each side of it with a same sized section of border fabric between them. Or a row of 3 or 4 of the cornerstone blocks starting left top and going down, then right bottom going up.
I really like doing this with kaleidascopes when I can then use some of the smaller scraps in the cornerstones.