Well, here's the finished seismometer ready for delivery. It was picking up the heavy surf off the Sonoma County coast for the last week, and also recorded a 3.0 at The Geysers.
To get here, I changed the upper suspension to be mostly rigid. It's hard to tell in the photo, but the angled part is 1/2" x 1/16" aluminum angle. I only have a couple of inches of exposed wire. This really killed a rotational movement about the boom axis that was cluttering up the results.

You can also see the cast lead masses at the end of the boom. I did these with a simple wooden form, and melted the lead on my Weber barbecue over some charcoal briquettes. I ordered the lead, a ladle, and some casting putty from Rotometals. The putty was very nice to smooth out the inside corners of the mold. Rotometals has a bunch of great stuff for low-melting temperature metal casting work.

The electronics for this setup all came from Larry Cochrane at the Redwood City Public Seismic Network. I bought a sensing coil, amp/filter board, and 16 bit A/D board all in a nice box. They are working great.