There are a couple reasons why I didn't put a slit on rod holders 1 and 3. Doing so, I would've needed to use two bungee cords instead of one long one. Also, I would have to make sure both bungee cords were the exact same length or the assembly may not sit flush with the tube. It seemed to be simpler if I put the slits in rod holders 1 and 2 and used one long bungee cord. Just attach one end of the bungee to one of the D rings on the bottom of the tube, run the bungee cord over the two slits, then attach the other end of the bungee to another D ring on the bottom of the tube.
As for the straps, I used utility straps that I bought from Walmart. They should be in the camping area. If I recall correctly, they were $2/ea. Attach one to each D ring on the side of the tube. Adjust each one to make it level.