Regarding boom material choices:

I have experimented with both threaded rod (1/2" aluminum),
various tubing, both round and square, and U shaped channels. 
My experience is that the rod and tubing are difficult to
work with, since they are round and/or hollow (=long screws),
and the threaded rod really limits the use of fasteners to large 
nuts and washers, which can be found in aluminum (or brass/stainless,
but these may add weight where it is not needed) if your True-Value
store has enough of those yellow specialty hardware trays.

But aluminum channel is available from most hardware stores, and
in a boom dimension of 3/4" wide by 1/2" high and 1/16" wall, provides
unlimited fastening opportunities wherever you can drill a hole.
As can be seen in the photos of the seismometers on my web site,
I use the channel open side up, which provides a place to run the
leads to the coils, drop trim/test weights to balance the boom, etc.
As can also be seen, I drill rows of holes in all three sides to facilitate
experimenting with the design-in-progress as the ideas come together.
The hardware stores (Ace, True-value, around here) have a selection
of sizes that often nest or fit together closely. A wide channel can be
forked at one end to fasten the mass in between, and forked at the mast
end to bypass the support mast so the lower hinge can be a short tension 
wire pulling from the back side (if the mast is narrow enough).

Without resorting to specialty suppliers like McMaster-Carr, there is also
a good selection of thicker (1/8") aluminum angle that can be drilled and
bolted together for the frame. For the base of the large horizontal made
up with angles and a flat piece (3" wide x 1/8" thick), I used filled
epoxy to glue everything together as well as bolting it (after
roughing all the surfaces with 200 grit sandpaper), and it made a
very robust frame in liew of buying thick aluminum  plate. And since
True-Value sells the 1/8" x 3" in 6 foot lengths, you can laminate
(with the aluminum-filled epoxy and lots of weight (park on it)),
several thicknesses with what you have to buy (or make 4 seismometers!).

As for the relative dimensions of a horizontal boom length to support 
post height, most designs have about a 30-60-90 triangular relation, 
with about 30 degrees at the mass/tension wire end or 60 degrees at 
the upper hinge at the top of the mast. Higher hinge supports that make 
up closer to a 45-45-90 triangle push the clearance height problem 
of providing a sealed cover for the instrument, and more acute angles 
increase the stresses on the mass/boom support wire and/or hinges. 
However, some strictly "garden-gate" designs using crossed flexure or
boxed flexure hinges have very shallow heights, like 6" for an 18"
boom.

Regards,
Sean-Thomas

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