I started by tracing the outline of the two pistols I needed on a scrap of 1/4" plywood, in this case, a S&W M1917 and a M96 Bolo Mauser. Trace the grip outlines ( both right and left ) on a piece of 1/2" pine, or other wood. Cut out the pistol outline and set it aside. Next, cut out the grip outlines, then contour them on a belt sander close to the final shape. Glue the grips on the plywood piece, then set aside to dry. When dry, finish contouring the grips on the belt sander, and finish up sanding by hand. Simulate grip screws by drilling a hole with a drill press, recessing both sides for the screw and nut to sit flush. Add a lanyard ring by screwing in a small eye bolt, then install a keychain ring or lanyard of choice ( the S&W ring was made from a piece of wire ).
The props are finished now except for stain and paint. Stain the grips in an appropriate color, then let dry. Paint the remainder of the gun black. Also, don't forget to paint the plywood black in between the grips - this simulates the metal frame the grips are attached too. Finish off with a light coat of varnish, or leave it a natural finish. I was originally going to carve in the checkering, but decided not to as it won't be seen anyway during filming.
That's it! Hopefully this method might come in handy for those of you looking for a cheap alternative to fill your holster.
Thanks, guys. 
The straight-sided Mauser grips went pretty quick, but the rounded contours on the S&W grips were a bit of a pain to shape corectly. I could only do so much on the belt sander ( especially the inside curve ) before having to finish it off by hand-sanding ( a drum sander would probobly work well for doing this job ). I made the recessed circles for the S&W medalllions with a Dremel as a test, but those didn't turn out as well as I would have liked - not flat and centered enough. If I were to do this detail again, I might try a sanding bit in a drill press for a better look. As it is, they can't be seen when in the holster anyway, so no biggie. 
