With a hollow back it is almost impossible to find a saddle off the shelf.
Whether a curved tree or a flat tree is used will depend on the severity of the individual problem The help of an expert saddler with appreciation of how to flock the saddle will be paramount. The panel needs to be full cut so that a banana shape can be made through the middle with less packing at the front and then the saddle balanced with the appropriate rear gusset. Otherwise the most problem is bridging in the saddle
Some horses are born with this conformation, others are working in the wrong outline for various reasons. One reason might be that the horse is working against its saddle and rider for reasons of discomfort, which may or may not be directly related to the saddle. Less than perfect shoeing causing imbalance in the foot can cause problems in the shoulder and have a direct effect on the way the horse holds itself, as can problems in the mouth with teeth or incorrect bitting.
BRIDGING is the term we most frequently hear applied to saddle fitting problems for horses with this conformation. Be certain that you are not confusing the problem of contact throughout the saddle by placing the saddle a bit too far forward so that the forward projection of the flap is resting on top of the shoulder.