Forming an upper triangular matrix from the elements of a list

I asked this question before, but i was closed because it the question was not comprehensible enough. So i reworded it, hope that's ok.

I have a list with 6 elements.

{a1, a2, a3, a4, a5, a6}

Now, I want to generate a upper triangle matrix with the elements above as matrix elements, so that the matrix reads

{{a1, a4, a6}, {0, a2, a5}, {0, 0, a3}}

That is, the triangle upper matrix matrix shall be filled up with the elements of the list. It is not important where each element is placed.

I want to do this also with larger matrices, so for example a list with 36 elements shall form a 8 x 8 upper triangle matrix.

Any ideas?

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