The solid is difficult to work with, but after about 8 repeated tries:

There appears to be actual bevel thickness on the delta side of the cab solid.
There are two copies of the cab. One has a skinny delta shape, which will not quite match up with the other cab.
There would be a narrow sliver of a gap, between the "wrong" surfaces, or overlap, near or at the delta shape.


First of all, color the box magenta and hide it.
Secondly, Separate the solid cab.
Pick the "inside" surfaces, one by one, and color them say red. Join the red surfaces.
If more than one red joined surface occurs, some of the surfaces are the wrong ones, with a narrow gap or overlap, so swap a surface at or by the delta,
until the red surfaces are "continuous", without gap or overlap, and will join to one joined surface.
Hide the red surfaces.

Color the remaining "outside" surface say cyan. It should Join to one surface, if the red one joined to a different one joined surface.


Now the magenta box will trim the red cab.
The red cab will Offset Shell just fine, either outwards, or Flip inwards.

- Brian