Why shape them? Put a thin layer of Bondo on the base of each skid
and have the Bondo conform to the wing shape by pressing it against
the wing exactly where you will ultimately put the skid. Make sure
you put a couple layers of Saran wrap or polyethelene on the wing
first, so the Bondo and skid will lift off after it is hard (a few
minutes. Sand the Bondo to conform to the outside shape of the skid
and paint the whole thing white, then remove the Saran wrap and bond
the skid to the wing as you were planning to: contact cement, double-
sided tape, whatever. Remember, though, that a strong bond will
result in the skid peeling off gelcoat if the skid hits hard -- you
might be better off making the bond with cement that is weaker rather
than more aggressive, so the cement gives up before the gel coat.

Fred