Why are tefillin batim cubes, other than because Moses said? Does anyone know or can direct me to who may know? Would you also cite your source, please? Thank you.

Shalom uvracha, Rav Shimshon Rafael Hirsch (in his book 'Mitzvot As Symbols' and in his commentary on the Torah Shemot 27:8) explains that everything in nature, such as plants and animals that build anything such as nests - it will always build it in a circle. The human being, on the other hand, measures the angles and builds square buildings. Therefore, the tefillin and the Keilim in the Beit Hamikdash such as the Aron Hakodesh, are all square and not circular, to symbolize the holiness and pureness of the human by his ability to create with his own wisdom and free choice. I'll just mention that the Rambam writes (Meila 8:8) that it is good and important to give explanations to all mitzvot. However, if one doesn't understand the reason for the mitzvot, he must realize that there are deeper reasons present. The fact that he can't understand the reasons does not mean that G-d doesn't have wonderful and even beneficial reasons. All the best!