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".
All,
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. - Jim