I have a Rocky Mountain horse that is exactly like yours, short and wide. I ordered her a Steele Custom Tree. Be aware, the Steele name is on 2 companies that are right down the road from each other. I think they are cousins or some relations. I'm not sure of any flex type tree because I am a heavier rider. I think they flex too much if too much weight is in them. I'm really loving my Steele Custom Tree and Saddle. The other name is Steele Saddle Tree. Very confusing I know.
I get the National Bridle catalog and have always liked the looks of the Tennessean.
If it doesn't work out, give Eric a call at Custom Tree and Saddle, they do a try before you buy deal. Trail Saddles by Steele