I am using multiple methods.
Straps with cam buckles across the top and bottom of the boards horizontally, then over and under the tire and vertically on the back vertically. Hope that makes sense.
Also have a rubber strap from bottom right to top left across the back, just as back up/redundancy, so to speak.
Then a locking Master cable through the two top holes horizontally and across the top to the back of the tire.
I've got thousands of miles and 2 years using this method.
2008 GMC 3500 Quigley Weldtec 4x4 Savana SMB
2006 BMW HP2 Enduro