After quite a bit of thought, I ended up using a pretty low-tech
solution that worked well for me. YMMV

I picked up 4 small metal cake pans for 50 cents each - the ones with
the removable bottoms and dumped the wheel weights in there. With both
the front and back doors of the garage open to provide ventilation, I
used my soldering torch pointing into the pan to melt the weights and
picked out the clips with a pair of long pliers.

The advantages of this was that it wasn't complicated and did not
require handling molten lead. The amount of heat involved wasn't
difficult to come by as I was really only melting the top layer. I was
also able to tear off the pans fairly easily to end up with my finished
weights (the pans were cheap). I suspended an eyebolt in the pan at
the beginning with a nut and washer on it so I have a way to tie the
weights down and transport them. It only took moments for the
completed weights to solidify.

The disadvantages were that it did take a fair amount of time to do and
the resulting units still contain a fair number of clips and
half-melted weights so they're kinda ugly and dirty.

Each weight comes in at about 15lbs and I ended up with 4 which is
pretty much exactly the weight I wanted.