By all means, add more lead to the process. I propose reclaimed ammo from firing ranges and other spent lead that requires disposal.

End result targeted should be solid, stable, and human safe to be next to.

Interesting proposal. Hadn't thought about spent ammo, but it's a huge problem. The issue there is safe transport and handling, but that's prolly a minor issue grand scheme. More interesting is the encapsulation tech needed to 1) block the relevant radiation effectively enough for it not to be a proximity hazard, 2) to be stable enough for long enough to see the dangerous half-life out, and 3) to encapsulate the lead as well as the radioactives. Not sure if glass encapsulation does that...again, there's science to be done here.