Securing the generator with either a stout chain or heavy duty cable, as well as a "bolt cutter resistant" padlock will do the job. Most thieves are after targets of opportunity and will grab the generator cleverly hidden under a picnic table, under a tarp, or in a pickup bed with no 'tether'.
Last time I went to Quartzsite I saw all kinds of generators with nothing securing them and some that had nothing more than a dog tether chain that could be cut with diagonal cutters.
Just remember, thieves will first go for the easy score. Then, if they really want something, you can't secure it well enough. For that event, there's insurance.