I know you were looking for a potty seat and thats not what any of us are suggesting, so I apologize. But for me...I do home childcare, and no matter what size they are...I use a potty seat as little as possible as its another thing for me to clean and move from here to the there and might drip pee or whatever. When I have up to 7 other children under kindertgarten age to work with at the same time, I need to remove those extras.
I can tell you that I have no children in my home over age 2.5 who are NOT trained and typically are closer to 2nd birthdays..so I get them all sizes. Boys almost always do the sitting backwards thing. It puts their hips are in a great position (or they can easily be taught to tilt them a bit) to pee downwards and they can see what they are doing (I encourage them to "make bubbles)..their bottoms are held up by the toilet seat so they don't feel like they are falling in, etc. Its a good progression to eventually standing up...facing the right direction already!