We have used the cheap landscaping timbers for beds. They are not holding up too well. Some poles are already rotten on a few of the beds. That's a waterline in the middle. After sowing some lettuce we put straw on top to help with germination and it tends to keep the lettuce leaves cleaner after a rain.
This is a cattlegate from Tractor Supply that is put in straight. We will probably put our green beans here this summer. This is one of our favorite sizes. I think it is 3 feet wide which allows us to plant a row on each side of the gate.
This shows our double cattle gate arch held in place with metal fence posts. These arches are nice for sugar snap peas and butternut squash, cucumbers, etc. We plant 4 rows of sugar snaps (one on each side along the gates) and then we have room to plant other bush type plants such as brocolli in the open area between the arches and the outside of the boxes. (The background of the first picture shows what you can get with leaf mulch and lots of churning with the tractor tiller. It is NOT Stoke County clay!)
We have treated 2x12's and 2x10's for beds that are 4x4 and 4x8. Some are stacked 2 high so that you don't have to stoop over so much. The taller ones also give lots of root space for things like potatoes.