O/K well tanalized wood is out because it (you might know) leaches into the soil . There are woods that are resistant to rot more than others , you would have to ask around what types there are in your area . You just box it up and staple the inside with black plastic .
Depends what you want it to look like , but you can build a dam of earth (about 6in high )around the garden , when you water you just flood the inside , a couple of inches and let it soak in , its a good way to irrigate a garden .
What are really good (if you are near railways , some garden centers have them ) are old railway sleepers < thats what we call them down here , They are the wooden supports that the rails lay on .....they will out last you .