I bought a pair of JJ Sinclair's wing tie down kit. http://cumulus-soaring.com/jj.htm

It is a piece of stiff and padded aluminum that captures the trailing edge of the wing and then has a strap that goes around the leading edge and from there to a ground stake. It works great.

It should work on your 18m tips as long as the ailerons do not go all the way to the very wing tip. For a "belts and braces" approach, add a wing stand.

My $0.02. John