I've noticed that many of the structures in the Mishkan as well as elsewhere in Judaism are cubes and squares, or multiples of cubes and squares.
The luchot are two half-cubes which combine to make a full cube (1x1 ama)
The Gold Mizbeach is 2x1 ama (or two 1 ama cubes [5tefach/ama]) with four cube corners
The Copper Mizbeach is 5x10 amot (or two 5 ama cubes*) with 5x5 tefach cubes on its' four corners. *The actual measurements are a mix of 5 and six tefach amas, which changes it's actual size (Eruvin 4a & Menachos 98a), but it's rough description is 5x10 amot, 250 times the size of the luchot & 216 times the size of the Gold Mizbeach, and a quarter of the size of the Kodesh Kedashim.
The Kodesh-Kedashim is a 10x10 ama cube
The Holies is 20 ama x 10 ama (or two 10 ama cubes)
The courtyard is 100x50 ama (or two 50 ama squared)
Tefilin are 2 individual cubes.
Tzizit have four-squares on their corners.
The Shulchan is 6x12x9 ama (or 648 tefachim cubed, which is three times the volume luchot)
The Lechem Panim were 5x10 tefachim and folded into 5x5 tefach squares.
The choshen was 1x1 ama squared
There were 50 letters on the Choshen's stones, 25 on each shoulder stone.
The Parochet was 10x10 ama squared
Was it just an easy shape or ratio to make?
Edit: I've been thinking it has to do with the "pattern" that Hashem instructs Moshe to build the mishkan after, which He showed Moshe on Har sinai.
"According to all that I show you, the pattern of the Mishkan and the pattern of all its vessels; and so shall you do." (Terumah 25:9)
"And you shall erect the Mishkan according to its proper manner, as you will have been shown on the mountain." (Terumah 26:30)