There are plenty of steep, twisting ravines.  If the wreckage were concentrated at the bottom of such a ravine, you'd have to be at exactly the right angle to see the wreckage.
Exactly right.  I've hiked in various parts of GCNP.  Those side canyons are riddled with steep, almost pit-like ravines that twist and turn.  With a small homemade plane, I don't doubt at all that the wreckage could be crumpled in one of them and be quite hard to spot.