For a helmet to fit in the frunk it has to be empty, except for maybe a couple of items that you could put inside the helmet when you stow it. To ride around with such a handy box empty seems to me to defeat the handiness of it. You can place the helmet on one of the mirrors, and with a small padlock you can lock the helmet D rings to the mirror stalk. Then the frunk can be stuffed with neat stuff which will bring you great joy.