Nobody knows how the original Shield of David looked like in the past: It was never found in any dusty excavation; It was never painted or drawn by eyewitnesses. There isn't any written testimony about its look. All we have are modern versions about how it COULD HAVE BEEN LOOKED LIKE; for example the rectangular model Uri Ofir made for his exhibition titled MikraOr; or Arthur Szyk's lithograph entitled Israel (printed in 1949) in which we see Bar Kochba sitting next to a big blue Magen David holding in his hand a round shield with a yellow Star of David on a blue round background.
One of the theories about the origin of the Star of David is that during the Bar Kochba rebellion against the Romans in 70 C.E. a new technology was developed for shields so that behind the shield there were two interlocking triangles.
The translation of the Hebrew name Bar Kochba is the "son of the star". (His real name was Simon ben Cozeba, spelled also as Shimeon Bar Kosiba).
According to Jewish messianic expectations the Messiah is an offspring from the house of David. No wonder that in the past when a Messiah presented himself to the people he connected himself to the Shield of David. That was the case with Bar Kochba; that was the case with the Southern Iraqi Jew David Al Roy (aka Menachem Ben Shlomo), who started in the twelfth century a Jewish crusade aimed at conquering Israel by force of arms. That was the case with Shabbatai Zvi (1626-1676) who adopted the Magen David as his principle emblem.
The next Messiah will ride on a white donkey and I'm sure that in his hand there will be…a Shield of David.