I would like to build them out of Cedar or treated lumber, which is best?

They will be on a Kentucky Cabin.

One of my projects this summer. Made them out of 1 by 6
cedar that I purchased as Menards. I used a miter saw and router with a 7/8's strait bit. The joints were glued and joined with 2 1/2" deck screws (thats what I had around the house). I learned the hard way that if nails are used over time the box will fall apart.

Start with your end-caps. With the miter saw set at 22 1/2 degrees cut that 1 by 6 with approx. 6 inches on the top side of the end-cap. You can make the boxes any length you want. After cutting the front and back board for the box I took my router and made straight grooves near the ends of the front and back boards to secure the end caps. You'll have to dry fit the box together now and cut and rip another cedar board for the bottom. When ripping the bottom board if you can put a 22 1/2 bevel on the front to follow the lines of the end caps.

I got fancy on the back side of the box and put a french cleat there to secure it to the shed and put returns on it to tidy it up. This way you can easily take the box off to work on the flowers or whatever you have in there.

One more thing to remember is as your making your cuts and putting the boxes together make sure the rough side of the board is facing out (another one of my mistakes).

Best of luck with the project, rredogg