Apparently I've forgotten how tricky learning the difference between 'b' and 'd' can be. This is a persistent issue I encounter with elementary schoolers I tutor. I don't blame the students — in modern fonts, b, d, q, and p are the same glyph reflected across an axis.
Naturally, 3D printing to the rescue. The files attached here are models of 'b' and a 'd' with arrow-shaped serifs. I've had the most luck explaining it as "make the arrow go with the word," i.e., we read from left to right, and aligning the arrow with the direction we read puts the character in the correct orientation.
Having a physical shape the students can lift will hopefully be a) fun and b) helpful for a problem that's fundamentally about rotating/reflecting an object. The downside, if this even works, is maybe a class of kids who attach arrows to their letters.