Holice

You can/could do two things.
First...you can use the stencil and mark from the corners to the center. And elongate or sqush at the center of each side. This way the sides and the ends would adjust the same and have balance even if the sides and ends would not be the same.
Second...cut paper the length of the side and end. Fold it in half and then again. Mark the stencil from a corner to one or two of those sections. If the stencil/cable does not alight to one of the folds you will know how much you have to adjust each motif in the design.. For instance if you mark to the first fold and the motif is 3/4 inch either way and there are 3 full cables in the section you need to enlarge or reduce each motif 1/4 inch. Don't adjust the corner, but mark to the center of the cable and move the stencil 1/4 inch either to enlarge or reduce to make it fit into the space. The sides adjustment may be different than the ends but the eye will not see the difference because each cable is the same. This may be shown on The Stencil Co blog under Co mingle. There are episodes from the Quilter'tv show titled Quilt As Desired. I don't ever recommend marking border designs before testing first for any adjustments. I would have written thus before but I was under the impression all the border was quilted.