The HT rudders look a lot like the white fiberglass Bimare A-cat rudders: These have composition foam top 1/3, corrugated (wavy- vertical) fiberglass middle 1/3, hollow bottom 1/3.
Unfortunately I have seen a number of the insides of these fiberglass rudders, but not the carbon ones that don't break. Bimare Boards do tho.

BTW- I left my 1 year old carbon rudders with water inside in the back of my car- in the hot sun with tape on the little holes. Do NOT DO THIS. The heads swelled. I had to compress them in a vise, drill and put my Hobie 16 Quik-Fit rudder cam pins thru the rudder heads so they would fit between the castings. Bad Juju.