With a western saddle, the trick I used is I put my saddle on a saddle stand and twisted the stirrups to were I wanted them and then took a broom and put it in the stirrups. And that holds the stirrups in the position I want them. I think it will work much better on a new saddle, my saddle is used and it did work just took a while.
If that doesn't work then you will have to dampen the back of the fenders and stirrup leathers. Be sure to dampen the entire backside of the fender. By doing only part way you are liable to get a water mark.