In The Princess Diaries 2: Royal Engagement Mia makes use of a wooden leg device (Herbie) meant to hang on her saddle to give the appearance of her riding side-saddle. This was meant to make riding more comfortable and less awkward. Her grandmother Queen Clarisse explains to Mia that "Herbie" is a centuries old idea and that their ancestors knew a thing or two. Was there such a thing in history for this purpose? If so would it have worked or been made out of similar materials (wood, leather, etc)?
No, there really wasn't. It's just a silly joke for the movie.
Medieval images of women on horseback don't show a highly developed sidesaddle, and in fact many show women just sort of sitting sideways on the back of the horse behind a male rider who is actually controlling the animal. The plate for August in the early 15th century Très Riches Heures, for instance, has two such female riders essentially sitting on their horses like benches and turning their upper bodies forward to engage with their male companions. There is one woman riding alone, under whom a small structure can be seen, representing some sort of saddle to support her posture. In the early modern era, a more complex saddle was developed that allowed a woman to sit fully forward, with her right leg bent on top of the back of the horse as "Herbie" does in the movie; this was achieved in large part with the addition of a protuberance she could hook her knee over, which would prevent the rider from sliding back around to the side. In the nineteenth century, a second protuberance was added to go over the bent knee, which helped keep the rider from sliding too far in the other direction, and helped keep her on her horse when she and the horse jumped over obstacles.
This is a rather short rundown, but it's difficult to give you an in-depth answer when the direct answer to your question is a very, very firm "no". My point is, saddles were constructed to allow someone riding aside on the horse without needing to secretly ride astride.
