Materials for building inexpensive light-weight large planter?
Animal watering troughs make for low cost and easily implemented "instant" planters that are (or start out) light weight for the strength they provide. You can cut down on the weight of the soil by putting in a layer of crumpled soda cans or plastic water bottles in the bottom. I like the looks of the plain galvanized metal ones, but you can paint them if you don't, or there are plastic versions that come in colors.