Bondoglass etc will not stay there very long. If you have to modify a fiberglass panel which was made using polyester resin, you should use either epoxy (West, System 3, MAS, etc) or vinylester resin (3M is what we use) Polyester resin is great for laminating panels and constructing pieces, but it bonds poorly to itself; its gluing properties aren't very good. Epoxy and VE are MUCH better and quite a bit stronger. If you use the right additives, they will fair pretty easily and take primer the same as polyester, so the final product won't show any difference where you built it up.
The body for my car had been around for several years before we were able to get it on the car, and numerous cracks had appeared. We ground those areas back and layered them with fiberglass cloth and VE resin, then sanded to shape, and then faired with VE resin and a filler. So far, so good. We had other areas where things needed to be extended or reshaped, and we used templates like suggested above to hold the shape until the part was set up and would hold shape itself. In one instance, we needed to lengthen a rocker panel cover- one side was made too short for the car. We sawed it transversely and then fixed the halves about 3/4" apart using wooden splints and self-tapping screws, plus cardboard covered with a release fabric to help us fill the space in. I strongly recommend the use of release fabric when you are doing jobs of this kind- the one we use is called "Peel Ply" and is available through West System dealers. It can be reused over and over again.
I have done repairs of this type in the keel of my 36' sportfishing boat using these techniques; some of the repairs are nearly fifteen years old and holding up fine. I formerly used epoxy for everything, but after using VE resin for all the work on the GT40, I may use that on the boat as well. It is great stuff.