Sometimes it can be a PIA to get body putty/fiberglass to stay put while it dries, especially on an edge. I've found a simple solution; just hold it in place with slick plastic like that from a blister/bobble pack or in the case, a section cut from a cat litter container. The putty/F-glass won't stick to it. I've also used it to repair trim items like side moldings with fiberglass with great results.
BTW: That's my DIY 12x16 paint booth and the front lid and DIY whale tail I'm working now. Car will be in next. The lil square hole in the wall has very strong fan inside it with an 8" dia vent tube out a window. Works great.
[This message has been edited by Francis T (edited 09-12-2013).]