Substituting a buoyant closed-cell
foam for the more traditional wingcase materials changes the fly in two
ways: the fly will tend to float and the wingcase can be made in a
multitude of colors, either blending or contrasting with the fly. The
ability for the fly to float off of the bottom of the stream needs to be,

The link you posted didn't work for me, but this does seem like a
good idea.

I tried making whole nymphs from closed cell foam, with the
same idea: weight on the leader would take it down, buoyancy
in the fly would drift it naturally, a tad off the bottom.
But the flies I made were too buoyant. Wing cases only might
be just the ticket.

My foam nymph experiments eventually led to the Marshmallow Nymph,
which is one of the best fish-catching flies I make. Marshmallows are
made with open cell foam, which soaks up water like a sponge......but
it still retains a few air bubbles here and there, which does give the
nymph
a little residual buoyancy--which is similar in concept to what you're
trying
to do.