These are designed to build up your fins on your FiberMAX rocket without adding much weight as each skin is only 13g.
It also adds quite a bit of strength to your fins.
They are made from 1/16in nomex honeycomb. You get two pieces for each side of each fin so a total of 6 pieces.
One side is covered in fiberglass and the other side is bare honeycomb to be bonded to your fin.
You can bond them before installation or after your fins are glued on.
You simply brush epoxy on the fin and apply the skins.
You have to be creative with the edges to fill the honeycomb voids but filled epoxy or simply JB weld works well.
Some have found 1/2 round balsa from the hobby store works well too.