Q. Can you make a triangular suka? (using two standing house porch walls at 90°, you place a third closing both, creating a right triangle)

A. Afarkasta De’Aniya (O.H. 2: 98) mentions that in principle there is no Halachik problem in building and using a triangular suka, however there are allusions and remazim in the traditional square or rectangular suka. One of them quoted in Chemdas Hayomim (Suka ch. 1,) is that the suka represents the letter “hey”. The letter that traditionally represents the Teshuva opportunities so essential on Yom Kippur, which is therefore followed by Sukos. Mareh Cohen (Suka 2a) writes that the letter “hey” is last on the word suka, where each one of the four letters alludes to the halachos and laws of the suka. The “hey” represents that the minimum of two complete walls plus a partial third one (tefach), are essential in the building of the suka.
Horav Shlomo Miller’s opinion is similar and when needed one can use a triangular suka.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a