I've been using a piece of hard stylofoam (about 1/2") thick, cut it to the shape of my Elmarit 28mm ASPH and tape a piece of duct tape on it for easy removal. The original rectangular lens cap is too expensive to loose while shooting.
Actually, I do not like this hood for the 28Asph. It makes it look too big. Prefer the hood #12589 for the Summilux35Asph(not FLE) which will not vignet even for Full Frame.