You didn’t mention the age of this horse, which would help. She does appear to be at that horrid stage of undeveloped and old-enough-to-start. I would take a little different approach. First off, I would forget about having a saddle made for her, as, once she has developed, you will likely then have to restuff or sell.
Second, if you have a dependable horse from which to pony, I would take time to pony this horse. Ponying her will allow her to get out and muscle up, whilst allowing her to see what goes on in the big wide world. Pony her up and down hills to develop shoulders and quarters. This will also help her “leg up” so there is less risk of leg issues as you go along.
Finally, I would lounge her. If you have a lounge cavesson, that would be ideal, as you can change the lounge line from above the nose, to a ring lower on the inside (the side on which you are lounging). Constructive lounging helps develop balance and suppleness. Start with lots of walk and trot transitions, add halts and finally halts to trot. Make sure you keep the tip of your lounge whip low and at the inside hind. The object being to move from the inside hind leg, as opposed to leaping away from the whip.
The fact this horse has no top line is causing her to go upside down. She may have a large movement, but she is not using so much of her back and quarters. It is hard to tell from your photos, except that the one showing her in hand shows no real “working” muscle – that she has got along simply moseying. If you try to work her into a frame at this point, you will get the saddle rubs, an uncomfortable horse, and possibly a horse who will give up trying for you. Trying to maintain a round horse will wear both of you out – she doesn’t have much to give. Help her build up by allowing her to develop naturally – pony or even riding her up and down hills, long straight trots, etc. She will develop the muscle, then you can bring it on and make what you wish out of it.