If you plant a box in the ground (you would need wood or some other material to block the roots) then you need to ensure that there is proper drainage. Without that, the roots will likely rot or be overcome by fungus. So you need to have holes in the wood. If you use plastic, it needs to be a strong, sturdy plastic or the roots will just break through it (taproots especially have suprising strength, and I've seen them actually push the tree out of the ground because the root was growing but had not room to do it in). I suppose you could try a synthetic burlap of some sort (real burlap will decay very quickly, which is why they use it), or really, you could plant the pot so that it cant be seen (which is kind of like what a wood box with holes in the bottom would be anyway). With a pot, you could dig it up every so often and do a little work on the roots to ensure that they don't get out of control and just grow through the pot (or the wood box with holes, or the plastic with holes, or whatever). If you are really way super beyond opposed to planting a pot in the ground, then you could try a big layer of heavy duty gravel, but that would be a whole lot of work, expensive, and would probably not work forever.
If you do decide to go with wood, remember that it will decay a whole lot faster when it is planted under the ground, with the moisture and the bugs and all those other things that would compost wood in the ground in nature, and will need to be replaced fairly often.