In Eastern Cosmogony, the Circle is the symbol of Heaven while the Square is the symbol of the Earth. This is the reason why Chinese coins showed a square hole in the center.
The shape of the tsuba had to be square, because it is from Earth, but as Yin - Yang or Yi-Yo, it should contain the opposite, which is Heaven. So the corners were rounded in four quarter circles.

I present here two examples of which the Tsuba 1 contains the TENSHI character in itself while the hole for the sword, as well as the seppa are only secured by one side. This did not satisfy me because it was not strong enough. So In Tsuba 2 I again repeated the character deprived of the upwards curvaceous tail because it inserts itself into the larger's one and symbolically the larger Kanji will represent the Heaven, while the smaller one will represent the Earth. In short, a dialogue between the two sides and a technically and artistically more balanced tsuba. Square tsuba width is 75 mm or 3 inches.

Here it is visible the interaction between the large and the small kanji. Heaven and Earth. So the final tsuba had to be visual and technically balanced as can be seen below.

Here is the finalized rendition of the tsuba in blackened steel rendered by Eric Litton
A side view of the tsuba for better understanding of the levels.
Eric Litton decided to make a small animation which will also allow for a good overall view
The finished work
Here I found what the customer has said about the commissioned work