When looking for a saddle to fit your horse with a challenging build, it is easier to transport the shape of the horse's back than the whole horse. A simple method of creating a cast of your horse's back is to purchase a box of plaster impregnated cotton gauze strips from your local art & craft store (like Blick) or order online and follow the instructions for use. There are a variety of sources.

It is not necessary to "line" the horse's back with plastic or petroleum jelly. Measure the strips into lengths which will be long enough to reach from one side of the horse to the other like a saddle blanket. Dip the pieces in warm water and spread across the horse's back, overlapping the strips. Apply a second layer of strips lengthwise, smoothing the ends into the layer below. Working quickly apply a third layer crosswise like the first layer. The plaster cast needs to harden on the horses back otherwise, the shape will shift and an inaccurate shape will be useless. Be aware that hardening plaster generates a fair amount of heat, so you don't want to make too many layers of gauze. After 30 to 45 mins, lift the cast off of the horse's back and brush off the plaster dust. It still needs to cure for a minimum of 24 hours, so DO NOT put the cast directly into a box and ship. It will lose it's shape and you will have spent your time and expense on something that will not serve the purpose intended.

Now, with a duplicate of your horse's back, you can take it to a saddle shop, private party or ship it to a saddler to make a custom saddle to fit your uniquely built steed.