Wow! This is awesome! You and your husband were very thorough. Most people do not take so much care of their horse's comfort. What tool did you use to shape the upper boards? I was going to shape mine with an old hoof rasp.
I am studying your photos closely to make my own. Why and how did you decide you need the lower pair of side boards? It doesn't look like they make full contact with the pony. Is there anything you would change about the design now that you've put it to use? How much do you think this pack saddle weighs? And what are the dimensions and type of lumber?