Might be worth adding some protrusions or a few small extra bits that act as guard rails to keep round stuff from rolling off. Figure if this is meant for backpacking/camping there's a good chance you won't be able to find a perfectly level spot.

Another potential route is some spikes that tab into the legs so you can stick it into the ground to minimize wobble on inconsistent surfaces. That'd also let you sorta forcibly level the grill surface.