When I started getting serious about SF gardening, I decided to construct some permanent boxes. Started with cinder blocks. Since the garden is on a slope, it was a lot of work, but once finished, very easy to garden.
This was a good start, but over time, I realized that the boxes were just too close to the ground for my aging body, so decided to build the rest of the boxes waist high so I could stand and garden. This has worked out VERY well.
As to construction, this is how I did it. First the supports. I dug 18 inch holes and cemented 4×4 pressure treated verticals along with side and horizontal supports.
Crossed braced with standard 2X4’s.
On to the boxes. I used 2″X8″X10′ in common fir. I cut two feet off each board for the ends. For the bottom of the boxes, I used 1″X6″X6′ redwood fence boards. Cedar is another good choice. To save money, I made the boxes 2 feet wide (outside dimension) so I could get 3 boards from each fence board. I loose 3 inches on the inside but that is easy enough to deal with. Everything is screwed together. I built the boxes on the supports. I used a standard 16d nail to space the bottom boards for drainage.
I stapled standard weed blocker to the inside bottom. Then I stapled 10 mil plastic to the sides to protect the fir boards.
Next up. Water lines. All of the boxes have their own water lines. Again, it makes tending the garden so easy. Two minutes of watering per box and your done.
There are all kinds of watering systems available. Check with your local hardware store or nursery and get growing!