We've built a couple pack trees in the past few weeks - one sawbuck and one Decker. It took some figuring for the first ones to get the Decker hardware and the cross pieces of the sawbucks to fit properly on the top of the bars while still keeping the configuration of the bottom of the bars the same as on our riding saddles. The way we figure it, the shape of the horse's back doesn't change between carrying a rider and carrying packs, so why should the shape of the bars change? But it did lead to some interesting challenges in building the trees.
The crosspieces are inset into the top of the bars and the grooves under them are routered out deep enough that the extra thick piece of leather a saddle maker friend gave us as a test piece can easily pass under them. The "feet" of the Decker hardware are also inset into the tops of the bars and covered with body filler so there is a smooth surface on top. This hardware was purchased from Herb Bork of Bork Saddlery Hardware.
On the sawbuck, the top of the bars are at different angles front and back but the sides of the cross pieces are flattened so they are flat where the box will come in contact with them, letting the box ride smoothly. Since the Decker hardware is identical front and back, the top of the bars have to be flat to keep the hardware square and even.
We purposely make the sawbuck bars a bit thicker compared to our riding saddles just for strength. But the Deckers end up a lot thicker on the outside edge, especially at the front, because the angle of the feet on the hardware is so different than the angle of the bottom of the bars.
At the back, it is the crosspieces of the sawbuck that are harder to make fit the bars since the bars have a flatter angle compared to the front. But the hardware for the Decker fits on the back of the bars better.
We haven't built a huge number of pack trees over the years - just one here and a couple there - but the we have had good reports back on them. We can make all the same changes in fit for pack trees as we make for our regular trees, including making them to fit mules.