It should not be too expensive, you need a buckle on a strap with a keeper or two. Do not make it too short. Measure from the top hole of your saddle strap, to where you think it need to be, then make sure there is enough leather to go through the keepers on the shaft loops. You can always make it shorter.
The bottom strap in this photo I had made to extend the backstrap on my harness. It was $9 at the Amish.