There are a couple of thing s you could add for greater stability for this project.

Instead of using just nails or screws to attach the legs to the base you could add dowels or if you wanted to get fancy you could use dove tails. I realize that not every one has access to a dove tail maker. But by far this is the best type of joint to make for something like this. But of course it is up to each person to add there own touch here.

This is a great idea something I might attempt myself if I can round up the wood needed.

You might want to think about using green or pressurized wood or at least seal the wood before putting it out side.