A lot of CL guys use dollys (or is it dollies?). Search their forums for some ideas, too. They are basically wire frames with a TE stop that is hinged. The other end of the stop is trailing under the airplane. As the airplane rises the drag wire releases tension on the rear stops and the dolly falls free. I've never built one but have seen them in action. Paint 'em real bright- they tend to hide in the grass!