What I did was drilled a 1" hole in the center of the 4 X 4's, I made the 4 X 4's 16" long, put the 4 X 4's on the Cabinet Clamps, the pipes WERE 24" long (I cut them for length after I knew where everything was going to set). I butted the setup where I wanted them to fit in between the tires, marked the back side with a black marker, where the 4 X 4's met the tires.
I cut them on my Bad Saw, but it could be done with a saber saw (jig saw) or reciprocating saw (saws-all).
Set them in place & looked at the fit, I think I had to fine tune one block.
Like I said they seam to work fine for me. I've tried the manufactured ones (Bro-in-Law has a set) I see NO difference in movement with either set.