I would recommend against using any fiberglass sleeve
that slips over the wing as a tiedown. They will tend
to pull on the control surfaces, unless you make them
so that the controls can move full travel within the
sleeve. And even if you do that, you will not be applying
the load where it is good for the wing. You will either
be pulling from the leading edge, or you will be pulling
in the middle of the bottom side of the sleeve, where
it will pull away from the wing and try to pinch the
leading and trailing edges.

Does your glider have a CG Hook? If so, make yourself
a short rope from 3/8ths hollow braid polypropelene,
or similar. On one end, put a chain link or a Tost
ring. Put a loop in the other end. Secure the loop
to the ground (through one of those tie-down rings
on paved ramps, or a good tie-down stake you put in
them move the plane over), and hook the other to your
CG hook. Push the glider back until the rope goes
tight, and block the wheel. Tie the tail aft to another
tiedown, and put stands under the wings.

As long as you rope goes 30 to 45 degrees forward of
straight down, you should not have to worry about back
releasing. Load to this towhook (for ground launching)
is generally between 80 to 100 percent of gross weight
for the weak link. So, the structure is probably designed
to handle at least two times this. And to get load
on this equal to two times the gross weight, would
require lift production equal to 4 to 5 times the empty
weight. (remember, the wing has to lift the empty weight
before it can start loading the hitch).

Pluses: No sleeves on the wings to load up your control
surfaces or hold moisture against your gelcoat. And,
have you looked at those sleves and gotten an idea
as to how much load they could really handle? I bet
most would come apart at a less than a couple of hundred
pounds. The tow hook is a designed in strong point
in the airframe, made to handle big loads. If you
put the tail dolly on, the angle of attack will be
low enough that with flaps set to negative, it would
probably take 100 MPH winds to even start loading that
tiedown rope, if it ever got loaded.

Just a thought.

Steve