Lettuce is very expensive at the moment. In some stores a head of lettuce is more than four dollars. A home lettuce bed could be a good investment.
With lettuce beds you are creating a garden off the ground to protect the plants from insects in the soil. You can also cover the bed with a screen to protect against animals and insects eating your greens.
With the wood for the bed you have two options: either you can use untreated cypress or cedar or you could use pressure treated and find a way to separate the pressure treated chemicals from the dirt that grows your food. The chemical used to pressure treat wood is much less toxic than it used to be, but it is best to keep a barrier between the treated wood and dirt in which you are growing your lettuce.
We used a moisture barrier that is used on roofs to wrap the pressure treated 2×8. We chose a 2×8 because it is the smallest two by something that was rated for ground contact. The moisture barrier should protect the wood from the dirt, but ground contact rating should mean it will last longer.
The idea is to build a rectangle. We used two eight foot boards cut into three foot and five foot pieces. A challenge is supporting the dirt. It can weigh a significant amount when it is wet. We used landscaping paper, which was supported by leftover horse fencing. To underpin the fencing we used rebar supported by brackets attaching it to the underside of the bed.
The legs were 2x4s with 2×4 supports run around the lower outside to prevent the legs from wobbling. The legs need to rest upon stepping stones or they will degrade quickly.