Good job, i'd say you got out of it pretty much what you were looking for - thanks for the pics from your site, this was a nifty little trick
But I have to ask - why make the 3d mold and pour the solder in from the top? - picture this for a sec:
You get a 'lil square box, not unlike the one in 'The Italian Job' as aforementioned...except maybe a little thicker and you do not need the top half...but you do need one side to have no ...well...side
(make sure to omit one of the short sides and keep both long ones)
so basically you have a box with a floor and 3 walls with no roof. (filled with clay)
place the key on top of the clay with the bow right on the edge of the clay, and evenly push the key down through the clay to near the bottom.
(optimally you would carve out a thin channel near the center of the key beforehand so that the clay didn't get pushed outward from the key as you depress it)
Remove the key very carefully, not nicking or touching either side of your depression.
(if done correctly you now have an oversized impression of the bitting ...say 4x to 5x the thickness of a normal key)
Now you can wall off the fourth side of the box you left open to make your depression, and (carefully so as not to damage the impression of the bitting!!) pour the solder into the mold and let it cool/harden.
Now you have a thicker 'fake' key that you can take to your key-cutter that should be a bit more resistant to warping or bending while you cut.
Also, it may make sense to only cut the key close to the actual bitting and file it the rest of the way to make sure you get a good working copy on the first try...i've never tried a solder key on one before but im fairly certain the key-cutter would not be gentle.
I was intrigued by the experiment and am posting this in an effort to contribute to a topic I think is cool - and am not in any way endorsing criminal activity. I know this goes without saying posting @LP101, but sometimes it helps to clarify and this seems to me to be one of those times.
By the way - what type of clay did you use? You seemed to get a good impression from it.