Tent stakes work great.

In firm ground, a long screwdriver works as well, espeically on a .40 size plane. After all, you're generating maybe 7lbs of thrust, max.

If you have doubts, get a fish scale, setup your idea, and pull test it. If you can pull 15-20lbs, you'll keep your small models put easily, even if it jumps against the strap. In fact, you'll probibly pull the tail off before the stake moves. Bigger planes need bigger tie downs, obviously.

The giant scale guys at my field use tow straps attached to the fence for their planes. Overkill, but the wide strap spreads the load on the airplane and avoids denting the stab.

In a pinch, I've sometimes used a screwdriver stuck in the ground as wheel chocks. On a tail dragger, the plane can still tip over, but it can't run away forwards. Not ideal, but better than nothing