A lot can be accomplished with a chunk of aluminum of the same thickness as the width of the trigger guard. Lay out the pattern on the aluminum, using your old one as a rough guide. Then you can cut out the general shape, leaving some "meat" for finish filing. You can either use a jeweller's saw blade in a coping saw frame, or just drill a series of holes, close together, following the outline, and then use a small chisel to break the webs.
Filing is often thought of as crude, but it is actually an art form. Take your time, and rough the shape out with a course file, then a finer cut file. Once it is basically finished, draw file with a very fine cut file, and you should be able to make it look as good as "store bought" - or better.