I built a few planter boxes, for our bamboo and some raised beds, but I'm not much of a carpenter.
For the long sides, I used fence boards which I picked up from Mr Plywood (in SE Portland). If I'm remembering correctly, these boards are 6 ft long, 3/4 inch thick, 6 inch wide, and have slightly rounded corners. They don't carry them at Home Depot.
For the ends, I used 2 x 6 clear cedar. I used various length wood screws to hold it all together.
Steps to build the planters with angled sides:
1. Cut the top board for one of the planter sides. Angle both ends. I used a 5 degree angle.
2. Use the cut board as a guide to measure the top board for the other end.
3. For the next level down, cut one end of your board, lay the board flat beneath the top level board, with the cut side aligning with the top level board. Mark the opposite side for the next cut. Repeat this process for all sides. I'm sure there is a better/easier way, but this method provided consistent results and all of the sides match up nicely.
4. Once you have all of sides cut (my planters had four levels), align the boards for one side flat on a work bench or the ground.
5. Cut a piece of scrap fence board, or whatever else you have, so that the scrap piece is slightly shorter than the height of your side. Make sure to leave enough room on the bottom for 2 x 4s support pieces and floor boards.
6. Screw the scrap board onto the fence boards to secure the side. I used one scrap board for each end, and two for each long side. This is for the inside of the planter, so make sure the ugly side of the wood is facing you.
7. Once all four sides are assembled, place one end and a long side upside down on the ground. Align the end and the side and screw them together. For a nicer look, have the short ends of the planter inside the long sides.
8. Attach the other end and side.
9. Measure and cut 2 x 4s for the bottom - this part is a little tricky. You need to angle the ends of the 2 x 4s so they fit correctly. Attach with wood screws.
10. Turn the planter right side up. Cut fence boards for the bottom, and place on top of the 2 x 4s. Attach with wood screws.
11. Turn the planter upside down. Drill a bunch of holes in the bottom for drainage.
12. Cut heavy duty weed fabric and use a staple gun to staple to the inside.
13. For better drainage, you might want to place the planter on top of some bricks, or pavers.
