There are many kinds of commercially available valve boxes, most of them are fiber reinforced plastic of one type or another. I've never seen one made by someone, everyone I've ever seen was commercially available. I don't think it's a good application offer wood if you're going to build your own, concrete would be much better alternative.
Why are you going to be that deep? Most sprinkler lines are only 6" +/-deep, I've never seen any deeper than 12". Going 2 feet deep like you are talking about is going to be awful lot of work and I don't see any advantage to it.
Why are you going to be that deep? Most sprinkler lines are only 6" +/-deep, I've never seen any deeper than 12". Going 2 feet deep like you are talking about is going to be awful lot of work and I don't see any advantage to it.