E track. I love the stuff. I buy sections of it and cut it up to weld individual pieces where I need them. I needed a few more strap points on my little trailer, so I grabbed a section of track and started dicing it up. I welded four around the outside, and I recessed two into the center. I cut the wood with a multi-tool and plug welded the track sections to the frame. Another neat trick; you can cut the rectangular shape into any 1/16”-1/8” steel to accept the clips. I did this in the sides of my dump truck bed. I also cut the slot into pieces of tubing and welded them to the rack for a few low-profile strap mounts. Just sharing ideas.