bnut pvc works well and holds up good to the stress of the shocks. If you want to get fancy alum. makes a nice spacer also. If I remember right I used 1.5" sch 40 pvc pipe glued in a coupling. One coupling is long enough to make 2 spacers when cut in half. Mine were 2" long and made the ride stiff. I would not go over 1" in height. A spring compressor will be needed to help with taking the shock apart and putting it back together.