I got this idea some years ago from Matt Madison, when he was growing his own greens for Findlay Market. The idea is to build a long box up and away from slugs and at a height that you can easily reach and a size that you can easily work in.
You can plant your greens rather densely and in several courses (I divide the box up into zones and plant each zone a week or so apart so that I have a near-never-ending supply of fresh greens to harvest. You can also create custom blends, which is worth it since you don't have to worry much about losses due to slugs. I've mixed beets and swiss chard into my greens mix before. The baby greens are very nice.
You should also amend your soil with some minerals to improve the flavor and nutrition of your greens. I'll be adding some "green sand" to mine.
I've used Pressure treated lumber here, as they no longer put arsenic in it. If you're still uncomfortable, you can use non-PT lumber, but you'll want to brush the boards with mineral oil to preserve them.
Here is the recipe:
2 saw horses. I'll leave it to you to buy or make them. Just make sure that they are waist high, as you want the box to be high enough so you aren't bending.
For the Box:
2 1"x10"x6' Pressure treated boards
2 1"x10"x27 1/2" Pressure treated boards
4 2"x4"x26" Pressure treated boards
4 2"x4"x6" Pressure treated blocks
42 (at least) 2" treated deck screws
8 (at least) 1 1/2" treated deck screws
For the bottom
1 8'x26" piece polycarbonate corrugated roof panel (metal will work too)
4 closure strips (wavy roof panel supports)
Cut the corrugated roofing panel to size with a circular saw (use a finish blade, if you can--make a test/practice cut first off the end). Measure and make sure it is exactly 6' or slightly less. If you're much off, you'll want to adjust the lumber sizes. Cut all your pieces lumber to size per above. Try to be exact, as it will make for a more sturdy box.
Next, attach the 26" 2x4 cross-members to the long 1x10x6' boards with the 2" screws. Start with one at each end and then 2 24" from each end. The 2x4's should be flat and you should screw through the 1x10 into the ends of the 2x4's. 2 screws each side should suffice. EVERY hole should be pre-drilled. 1x10's tend to split easily.
Screw the end side 1x10x27 1/2" panels to the side cross members with 5 screws. Again, pre-drill every hole.
You should be able to set the box on your saw horses unless you are making your own legs. This will make the next step easier. Center the 24" closure strips (wavy supports) one on each of the cross members. Set the corrugated roof panel into the box so that the sides of the panel curl up, not down. The fit should be fairly tight and the panel should rest on the closure strips.
Reach under the box and feel for the closure strips on top of the 2x4 cross members. If they are reasonably centered, use 2" screws to fasten the panel in two places to each cross member through the closure strips. You should be able to eyeball this and feel to confirm that you are drilling into the closure strip. You want to drill through "high points" on the roof panel so that you do not over-penetrate the cross member.
Attach the 4 blocks to the corners using 2 1 1/2" screws each to the long 6' panels first. Make sure the blocks are flush with the top of the box and the end of the 6' panels. Then, screw the end panels to the corner blocs with 2 2" screws each.
This should be a very sturdy box. Before putting dirt in the box, you'll want to drill some holes for drainage. I used a 5/16" spade bit. I put a hole in each trough at each end, and then a few alternating holes toward the middle, just in case. See the pictures below.
Close up of the end holes.