Red and black composite carbon fiber(black fibers)/fiberglass(red fibers) over a foam core.
Blue and black composite carbon fibers, same as above.... but it's a 3 piece grip and it's not on the rod yet. The pieces in front and back of it are also foam core, but they have surf shrink grip over the top.
The white foam core is just shaped, and has not been sealed yet, it will have the black and red rubber shrink grip put over it. The foam cores are lighter than cork or EVA foam so the rod will stay light.
The fly grip is all cork. from the top down as an example....
I thought about stacked leather...but it would be pretty heavy. Not so much I guess once bored out. I think the wrap would hold up finished with bees wax instead of poly..really like the look..like a stacked leather knife handle. But you're the artist here ...I'm sure if you ever decide to play around with it..you'd do a good job.