According to the Talmud in Bava Batra 14a, the Luchot HaBrit were square. As a side note, I have seen those "correctly-modeled Luchot" in the Shteblach of Meah Shearim.
Where did the common "tablet" shape come from, with rectangles and a rounded top, like those that we see it in Synagogues all the time? Is there a source for such a shape, or was it borrowed from Christian art? If the latter, should we try to replace them in our synagogues?