A modular cube made from six double arrowhead units. It is based on Francis Ow's "Russian Puzzle Cube", but I use a different method to fold the units. I think my method is simpler, and it also shows both sides of the paper, which allows for some nice effects. Be warned -- assembly can be a little tricky, as each unit is held in place by other units, so it tends to be unstable until it's all together.
Each model requires six sheets of 2:5 ratio rectangles. These can be cut from kami or printer paper (such as Astrobright). A 6-inch (15 cm) kami square with a 1/5 strip removed can be cut in half lengthwise to provide paper for two units. For Astrobright, which is thicker (89gsm), 3 inches x 7.5 inches (or 8 cm x 20 cm) works well. Larger and smaller rectangles also work, but if units are too big or too small assembly more difficult. A bone folder is recommended, and paper clips or dolls' clothespins maybe be helpful. Tape, if it can be removed without marring the paper, is perhaps more helpful.