IHW - my mare's got the flattest back in the world xD so I feel your pain. Unfortunately I'm going to need to get her a treeless saddle - even the widest wintec gullet is too tight on her :P
Have you looked into some quality treeless options?
Also - please don't take this critically, a saddle is better without a pad than with a towel. '97 was 15 years ago, we've come far in equine welfare sense then. The reason towels are not good is because they can bunch up and cause pressure points under the saddle. It can also rub the hair backwards or pinch. Being broke myself I've found it quite easy to make saddle pads Just go to a fabric store and get some fabric, and some quilt padding and sew it all together! It's much cheaper than $50 pads x.x But if the saddle fits appropriately you're better off without a pad at all - many people do that the first few rides in a saddle anyway to get the leather soft and supple.
Another cheap fix is to just go bareback :P the lucky thing about wide-backed, hard to fit horses is that they're really freaking comfy bareback horses!! Hehe ^^ My mare's like a couch!