What i did was got a 2x6 that was around 16 feet long. and then i had that 2x10 on top that i cut half way to make the down part. Then you just mount the 2x4s to the bottom of the top board and to the sides of the bottom board. Where i made the kink for the down i used another 2x6 and cut the angle with a jigsaw so that it would keep the 2 pieces real close and would be stable. for a long flat down box i would def use siding though so that its more stable (depending on how high you make it)