Hello, this is my first post and i would like to say, awsome game

Ok, to the problem. I am currently trying to design an aquatic variant for a custom race i am designing atm. However, i cant seem to make a hollow fin, http://i92.photobucket.com/albums/l33/Veronw/HollowFin.jpg, a small example of a type of fin im trying to make. Any help would be greatly appreciated, as i havnt been able to figure it out over the past two weeks of trying.

Thanks, Veronw