In order for the playfield to lay flat in the box, you need to add ledges on each side for it to sit on. This holds it up high enough to keep the parts on the underside from laying on the bottom. Again, this is just something you'll have to measure for yourself to see where yours should go. I believe mine was about 2.5" from the top. I just used a couple scrap pieces of wood I had leftover getting the sides cut. Marked their position, glued them down and screwed them on.
This is a good time to lay your playfield in to check its fit.