This is a simple triggered spark gap I made, solely to see if it would work (I've never tried a triggered gap before). The electrodes are two M6 dome nuts (stainless in the photos below, but I've also tried brass). They are mounted on M6 stainless threaded rod so are adjustable. The trigger electrode is a 1/16" TIG tungsten inserted through the side of the housing - it, too, is adjustable. The housing is a piece of 25mm acrylic tube, with end plugs turned from acrylic rod. Two hose nipples (the grey things) provide a means of blowing air through the gap to keep it clean. The trigger electrode holder and the hose nipples are made from PVC and are epoxied into the side of the housing.
The housing does a pretty decent job of damping the sound of the spark - a 5J discharge at 10kV makes a satisfying pop, but not nearly as much of a "crack" as it would be in the open air.
Sizes aren't critical, the main thing is to demonstrate the construction.