How to sweep maintaining parallelism to XY plane?

[ct.rhino (Charlie T)]

I am sweeping geometry (square) along the edges of a larger (hidden) extruded polygon (triangle).

I would like to maintain parallelism to the xy plane but the square sweeps along the curve direction.
unnamed.gh (27.7 KB)

[inno]

this might be one way:

unnamed (3)_inno.gh (27.0 KB)