In order to get an accurate weight and balance tail of the glider must be raised to the flight attitude.
For the Schreder HP series the seam along the tail boom is a good reference. If this seam is not visible then consult the blueprints and compare the height and spacing of the tail-cone bulkheads to determine tail-cone taper.
Once the taper is known a wedge can be constructed similar to this.
As you can see this isn't a complex.
My wedge consists of a 4 foot level, a spacer with an angle at the top and a "C" clamp.
The HP-14 requires a 2.5 inch space.
This is a "no-brainer"; however, I'll state it
anyway. Be sure to remove the wedge prior to weighing the glider.