Thanks for the pics. I ended up making mine out of the AZ Rock
"Sand House/Beach Sand". It looks very much like yours. I like
my cars fairly heavy so I just built the entire load out of the AZ
Rock product (no "filler") ... the car is still not "too heavy".
Doing it the way I did it I ended up having to deal with
"gutters" along the sides and ends where the sand met
the walls. They were about the width and depth of the
end of the typical hobby file (approx. 1/8th"). I think
the gutters were the result of the glue+water 'filling'
those areas and then drying and leaving a void.
Doing a final application of the AZ Rock material - and
being careful to hold the car at an angle and tapping
the side so the sand filled the gutter resulted in a very
nice looking load. Michael did a much better job of
making the car itself look 'right' than I have (so far).
I consider these Intermountain composite Caswell
gons to be excellent models.