I decided to make the undercarriage out of common redwood 2 by 6s. I decided this because I have it just laying around. I designed a deck for a friend and helped him build it. It is the curse of the handy that friends come around when they need help with a project. They have another set of friends they ask when they have tickets to the basket ball game. The plus side is that he let me keep the extra wood. I happen to like redwood and the only things better than using redwood is using redwood you got for free. I think it will go well with the LVL top. You can see from the photo that I have four legs and four stretchers all roughed out.