With my Yamaha I drilled 2 holes through the frame below the handles, then used 2 heavy locks with heavy chain.
With the Honda I removed the plastic covers on both ends, removed the handles, then attached heavy chains to the frame through the holes where the handles had been mounted. Put the plastic covers on, threading the chains through the cover.
In both cases the generators were/are mounted on a cargo tray on the front of the truck, and the heavy chain was locked to the tow hooks on the truck. The generator was placed inside a custom box that let me fuel/start/connect, which also provided some out-of-sight-out-of-mind security when it wasn't running (and protection from rain and dirt on the road and rain while it was running).
2008 FR Salem 26TBUD
2010 FR R-Pod 171 (2009-2015)
2010 Chevy Suburban
Honda EU3000is inverter-generator