I never took a very detailed look at the SME 3012 because for ME it looks to fragile ( i preferred the Fidelity Research 64S). But if you have a steady hand it will be perfect.
If you look at it with some '' physical laws '' in the back of your mind some ideas will pop up.
You have the DL103 which is a nice cartridge which will work better with the lead housing designed by the French decades ago. Using this housing you will probably need a heavier weight. Of course the best thing to do is get the weight as close as possible to the bearing.
What I would do is check the maximum diameter of the weight that will fit in your set up. Then make its weight have enough to be able to mount it close to the bearing. Copper has 15% mass than steel so that could be a nice option too. I think this kind of thing can be machined by any student from a technical '' fachhogschule ''
There has been a British company that sold a contra weight as a modification to a British tone arm where the center hole wasnt in the middle telling us it would create a better functioning of the arm because the '' center of gravity '' would be below the arm. BUT it could be harder to have equal weight in the horizontal plane.