The IMI universal mentioned above would work perfectly. An ASW20 is actually fairly "modern" and the new wings are only marginally smaller. But if you read the description this has been thought through and described.

The following paragraph comes from the used IMI link above. I'm assuming you didn't read it yet (last paragraph of the description)...

"The universal one man rigging system (OMRS) WORKS WELL ON ALL 15/18 meter gliders. It accomodates older airfoils where the modern 15/18 m OMRS will not. The OMRS for modern 15/18 m gliders has a flatter holder only working with modern airfoils. - See more at: http://wingsandwheels.com/imi-one-man-rigging-system.html#sthash.zs8eUGv7.dpuf"

Good luck.