A friend of mine has wide horses. Her Shire x Paint (more Shire built than Paint tbh) and MFT are fat and round/flat backed with no withers, requiring an extra wide saddle. She rides in an Isabelle and a synthetic Wintec dressage saddles, both with extra wide gullets. Both saddles fit the ShireX with no problem at all.
Some Wintecs have interchangeable gullets too, so if you can find one (nicely) used in a 16 or 16 1/2 seat, you should be fine. I find the Wintec dressage saddles to be really comfortable compared to many of the jumping and A/P saddles I have ridden in, so I have no problem doing trail riding, arena work, and jumping in the dressage saddles with my small build. I ride in a 17-18 inch saddle just for the sake of my long leg. I'd probably fit in a 16 inch otherwise.
Think of it not as a failure but as a success in how not to do it.
Don't look in a horses mouth for a gift.