I have a 17 inch English saddle on a wide tree if your interested. It still has the tags on it. Leather Regency saddle. It's not great, but may work for you. Let me know if you are interested and I can try and get pics. Selling dirt cheap, $100 plus shipping.
SaddleOnLine gave great advice. Horsesaddleshop templates. They are downloadable and printable. According to the shape a coat hanger created it fits the extra wide. I shall transfer it to cardboard and see how that works.
There is a new Flinger at our barn that my daughter rode this week. She used her Crates barrel saddle on him. Someone else used a Crates Reiner on him. Both are equifit trees, FQHB saddles. He's super wide too, and they both fit with room to move his shoulders.
I have an amerigo vega monoflap jumping saddle for sale! It's a wide tree (measures 5"). Great condition, made of high quality italian calfskin leather!! Has long billets. I NEED to sell it asap. They're new for $4,000.. but I'm asking $2,200 but VERY open to offers!!!!