There's not really a great solution specific to the rack. I generally just wrap something scary looking around the bikes if I'm grabbing some food and drinks after a ride. The long chain locks look pretty intimidating and they have enough length to wrap through two frames and around the rack. The ones from Abus are reasonably priced. Or a 5 ft Schlage with your own padlock.

If somebody with an angle grinder was really determined, there's nothing stopping them. I feel like I'm mostly going for show to dissuade opportunistic theft. I would think a $25 combo cable lock from any bike store would have similar effect.

Like dangle said, you're not gonna stop the determined thieves.
I'd suggest getting a long enough cable and a pair of padlocks. Run the cable through the wheels and frames and padlock each end to the safety chain loops on your hitch.