I do not know what radar band ATC uses, but couldn't one suspend a
metalic reflector like sailboats use to provide a radar return? Of
course, space would be consideration depending on the glider model (see
http://www.tri-lens.com/trilensweb12002002.htm)

... Incidentally, we (USAF) did tests with fibreglas gliders to see if
we could "see" them with ATC radars.. you typically could not unless
they increased their power above the rated power they were authorized
at the time... yet, if we put wadded up tin foil (Reynolds Aluminum
wrap) in the wings, they showed up like gang busters.

Jettester (UP)