Want to build a barge to go with a Springer tug? Modular barges are truckable, that is they can be brought to a site, unloaded, fastened together to make a larger barge or bridge of almost any configuration. The real ones come in several sizes, 10 X 10, 10 X 20, and 10 X 40 (feet), X 5 feet high, are the most common sizes. A variety of accessories can be added.
Mine is actually one barge scored to look like four 10 X 20 barges tied together. It is made out of wood (34" sides, ends, X 1/4" top and bottom with some weight added internally.