The simplest way would be to start from scratch with a Block statement.
Something like:
! set variables to standard parameters (otherwise create user parameters)
beam_lg = A
beam_wd = B
beam_dp = ZZYZX
! model of beam
Block beam_lg, beam_wd, beam_dp
Plane 4,
0, 0, 0,
beam_lg, 0, 0,
beam_lg, beam_wd, beam_dp,
0, beam_wd, beam_dp
Plane 4,
0, beam_wd, 0,
beam_lg, beam_wd, 0,
beam_lg, 0, beam_dp,
0, 0, beam_dp
The Plane statements form the "x" in the beam section.
Rect also works, but requires rotating the coordinate system, as follows:
Block beam_lg, beam_wd, beam_dp
rotx atn(beam_dp/beam_wd)
Rect 0, 0, beam_lg, sqr(beam_wd^2 + beam_dp^2)
del 1
addy beam_wd
rotx 180 - atn(beam_dp/beam_wd)
Rect 0, 0, beam_lg, sqr(beam_wd^2 + beam_dp^2)