This is a side view of a Lehman style Seismometer I am building for a friend. I found some great base material at Bataeff Salvage here in Santa Rosa. It's 1/2" plate aluminum, about 6" x 18". The vertical section is 3/4" galvanized pipe and fittings. Here I'm just starting to figure out where the sense coil and magnet will go. I wrapped some plumbing solder around the end of the boom as a temporary mass. The pivot is made from two pieces of 1/4" round carbide. They are epoxied in place, one on the end of the boom and the other on the bottom of the vertical mast. The angled aluminum piece is there to hold the boom if it slips off the pivot. Only friction holds it in place.
One aside. Normally, as a good scientist, I prefer to work in Metric. But, scavenging for parts in the US generally results in inch based parts. Now, if a piece of pipe was manufactured as 3/4", I'm not going to convert it to metric. Use your brain if you need to find a metric size somewhat near the inch size I said I used. Besides, none of this stuff is that critical - use what you can find like I did!