I'm building 4 6'x3'x11" frames right now, here's what I'm doing
Cedar fence pickets, 5 1/2"x 6 ft. Stacked two high (11" total height). Each picket was $2.52 from Home Depot. Bought $10 worth of 2" exterior screws and a couple 2x4s as the vertical supports similar to what you see in this guy's video below. Used miter saw a little. Total cost is about $75.