Strong glue kind of Epoxy-Araldite or SuperGlue, attach 2 small hooks
directly to the glass (3mm glue spot).  Hung your board directly to the
hooks (holes at each top corner of the board). Some of those glues can
be easily removed from glass using a sharp rasor blade, some don't.
Customer can use the hooks to hung something else later...  or you can
charge them to remove the hooks. :)