Though this was stitched on a longarm, the fill can easily be done on a domestic machine. Originally found in Charlotte Warr Andersen's One Line at a Time Encore book, it's definitely a winner of a fill!
This fill requires a drawn grid (1 inch squares suggested). Start the stitching in one corner of a border square with a back and forth doodle filling up the space.
The next row starts in the other direction, at right angles to the previous stitching.
Another line, parallel to the first, can be started next.
Continue adding back and forth doodle lines following the steps above.
And in no time, there's a beautiful woven background fill