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I think you will find that your pony will be easier to use if the top part of the clamp is angled down at about 45 degrees (think roof of a house). It will give you much easier access to the workpiece when sewing and using the awl... The sewing gods also dictate that anything your thread can catch on, it will --- while it may seem like a good idea, the hooks on the pony will most likely just be thread snaggers...
Overall, not a bad design...
I would also suggest watching a few saddle stitching videos on YouTube --- while not "Rocket Surgery" it can be a little frustrating if not done properly... Good luck and have fun!