Bluraven is on the right path. You probably can find a stock one on fleebay cheap and cut some of the foam down.

 

Could she wear boots with a thicker sole? Perhaps a peg that she could hook her heel on would be better.

I have a drivers backrest....you better have one skinny wife for her to be able to sit behind that, they don't leave much room on a softail.

 

Joe G has a good suggestion when it comes to foot-wear. What is she wearing? I have riding boots with 2" soles and even a higher heel. I can't comfortable touch the ground otherwise. If she has two inch soles and heels then the only other option is the adjustable mounts. Seems to me, considering what we spend on our bikes, paying this bit for your wife's comfort is a small thing. (I know, it is easy spending someone else's money lol)