Starting with 1/4" x 4", 3' Oak. And I cut it in to two 9" pieces for the front and back. Then I cut the sides by cutting a 4"x4" square on the angle, then cutting one of those angles on the angles. I also put a 45 degree angle on edge of the front piece.    

I used super glue to hold everything together because I couldn't not come up with any way to clamp this together.

Went ahead and drilled and countersunk the mounting holes before gluing on the back.

Sanded a few problem spots flush, finished with a redoak stain.

Followed up with a few coats of spray lacquer. 

Finally I epoxied the entire inside to prevent any possible leakage.

I experiment with using masking tape to prevent epoxy from getting where I didn't want it, but that just resulted in epoxying the tape to the board. 

I don't have a final shot of the planters I built in this log, but here's a shot of the prototype I made a few months ago before I had any power tools, so it's much more rough, but you get the idea!