This may or may not have been posted before. When I used to do bottom jobs on the different boats, I had templates I made to match the widest part of the hull bottom. I tried several different materials, and came up with the plastic from a kids toy beach bucket as working best. It needs to be stiff enough to hold shape and pull a bunch of thickened resin, but soft enough to be flexed, and still hold shape. I cut it to shape and smoothed out the edges with the light touch of a propane torch. It took some time fiddling with it to get it just right, but they were used many times and I never wore them out.
You can also use it as a guide by flexing it along the repair, to gauge what needs to be cut down some more if you build it up too much.
After building up the bottom to the point that you need to fill the last little bit back to shape, I'd mix up some resin with silica thickener, trowel it on, and starting at one end beyond the buildup, flex the plastic template so the upper parts fit against the hull, and sliding it down the line, allow it to gradually flex out to the widest part of the hull and back in as you go narrower again, using the good part of the hull as the guide.
I just thought about the possibility that someone might have the templates posted online now, and found them quickly.http://home.earthlink.net/~mattson/hobi ... /keels.jpg
It saves a lot of time with final sanding.
I also made a set of four sawhorses that put the bottoms at a good working height that were time and effort well spent in making.