This is not an easy job. It also takes several hours to do. There is no good way to do it by hand that is cheap and easy.
The front of most handgun grips is not a perfect radius, it's an oblique. THis makes it very difficult to program CNC type equipment to work with. Given the fact that, due to polishing and other processes, the radius changes from gun to gun, you compound the difficulty.
Cutting and welding in a piece is fraught with problems also, as you then have to deal with changes in metal alloy, weld marks, clean up, and other things which show up when a finish coat is applied.
One thing often overlooked is the fact that you will have to have the frame refinished when the work is done. This requires the gunsmith to polish the required surfaces after he has removed the old finish and done the specified work. This also is time consuming, and costs money.
Basically it's like this...You want custom, you pay for custom. You don't want it enough to pay for it? Then you don't really want it.
As noted you could do this yourself. However, there is a fairly steep learning curve. Since the average trainee wrecks the first 10 (or more) mainspring housings, and they are much easier to checker, I suggest you plan on devoting time to practice. Nothing looks worse than a poorly done job of metalworking, and (trust me) everyone will notice if the lines are crooked, or the points on the diamonds are not uniform.