Here is a couple of tarped loads that can be used for gondolas or flat cars. They are really quite easy to build and only took about an hour not counting time to dry. They are made from scrap pieces. The pieces are glued to 1/8" hardboard cut to fit the width of the cars. You could easily use .04 or .06 styrene if you want to lower the load, it just needs to be stiff enough to take in and out of a car. You can easily hide weight in there if your cars need it.
This is a 40' load. Those are the little blocks it was sitting on. The kleenex is dry and I have trimmed off the bottom.
Next I took the load and set it on the floor with wood block spacers under it to hold it up. You want the spacers to be narrower than the hard board. It only has to be 3/8" off the floor. I put down a piece of plastic just to make clean up easier. I then covered the load with a piece of kleenex letting it rest on the load and pushing it down between the scrap pieces, then sprayed it with a white glue mixture and teased the kleenex to get some extra wrinkles in it. You have to be careful at this point because you can tear the kleenex but it is only a piece of kleenex so not really a big deal. If you have painted the scrap underneath a small tear might look really good. You must let this dry for 2 or 3 hours but once dry the kleenex is rather tough and can be handled. I then took a pair of scissors and cut the kleenex even with the bottom of the hard board. Now off to the paint booth, pick the colour of tarp you want and have at it. You could also paint it with a brush to get some extra texture and do all of your weathering by blending paints. Once this dries it can be weathered with some chalks to highlight the depth of the wrinkles. This is one of those projects that you really get to practice painting and weathering on. If you are not happy with the end result you can just chuck it in the garbage as it has cost almost nothing to make. You can use this same technique to make a tent or one of those new buildings that are just covered in vinyl sheet. If you want a shell I would put a layer of Cling Wrap between the kleenex and what you have under so it doesn't stick to it. I would like to take all the credit for this but Ray has been doing it for years and turned me on to it. He works in O scale so you can use heavier coverings if you like.
This is after it has been painted and weathered.
Here is the other load for a 65' gondola. I tried using thread to show a tie on the tarp. I have come to the conclusion that it is best added after the tarp is painted. I thought that the thread would stick easily to the wet kleenex but it did not. So I used brown thread sliding it through white clue and then glued it on after it is painted and before it is weathered.
Here is the load painted and weathered and the thread added after it was painted.
Here is a picture of Ray's O scale tent. Every item in this picture is scratch built even the figures. The car is a kit but has been heavily modified. He has such artistic talent.