Do you get many slugs or snails in your area?

If so, one suggestion is to put a copper strip on the edge of the planter box, since slugs and snails don't like crossing copper, which if I remember correctly is due to it causing a reaction with their slime. In the past I've used copper stripping made just for planters, which is kind of like tape that you stick to the side of the planter box.

Slugs and snails are the greatest pest/problem for my year round lettuce garden.