I stopped by OSH tonight and bought some 3/8″ 16tpi, 1/4″ long threaded plugs to use as stops in the tie-down brackets. The tie-downs that Van’s provides can screw in until they contact the wing skin and then crush it. These plugs provide a hard stop for the tie-downs to tighten against.
Here you can see that they simply thread into the tie-down brackets.
The lighting is poor, but you can see the plug deep inside the bracket. The tie-down rings thread into the bracket and stop against these plugs with about 1/8″ clearance between the tie-down ring and the wing skin.