You could paint the insides to waterproof it but I think in time, water will seep through and eventually cause rot. You can line it with pond liner or tin.

Perhaps you can look around for plastic containers that are about the same size, or slightly modify the plans for the wooden shell. (Which essentially becomes a cachepot.)

I'm thinking of the large Rubbermaid totes, for example. (I bought several of the 18-gallon size for planters for my upstairs neighbors porch.) If you keep your eyes open, or just cruise the hardware store's storage section, you may find something perfect.

If you use the plastic bins to contain the soil and plants, I don't think you'd need to build a bottom to the planter. Seems like you'd still have the potential for rot there, with water run-off.

As for plant selection.... with a lot of sun and a lot of wind you're going to have some pretty dry plants. Unless you are going to water every day, you should start by looking at drought tolerant plants.

Styrofoam, or that blue construction foam. That's what I did. 2" thick, it won't break down, it's great insulation. I cut huge rectangles out of the bottoms and covered them with hardware cloth under finer screen to keep the soil in.
Mahagony is perfect outdoors. It lasts a long time and turns that beautiful silver.