Welcome to the forum!
Based on your measurements, I would suggest a 15" seat. You could possibly go a little smaller but would be better off in the 15. As for your horse, everyone's idea of what is "wide in the gullet" is different. The absolute best way to try a saddle for fit is to take the horse with you. If that is impossible, then this way will get you close enough. Here is the way I suggest to everyone on how to measure your horse for a saddle:
| Go to the hardware store and get about 4’ of 12 or 14 gauge house wire (the kind electricians use to wire a house) and cut it in half. This was written for Western saddles but the principle is the same for English or Australian. |
Take 2’ and shape it over your horse’s withers. Take the other 2’ and shape it over the center of his back.
Carefully take the wires and trace the inside of the wire on a heavy piece of cardboard (or poster board if you have it). Cut out the cardboard shapes and take them to the saddle shop to fit against some saddles.
This part is Western: Keep in mind that QH bars in one saddle may not be the same in another brand. There are no standards for saddle trees so each manufacturer has his own idea what dimensions make the designations.
As for you, your bum should not be squished against the cantle, it should have a little room at the top and there should be about 4" of space between your tummy and the swell.
As for a rough out saddle, I've never heard of a synthetic one. In fact I can't imagine how that would work since the synthetic parts are always the seat and fenders.