ASTROD Spark Gaps
Alan Trent knows of many different types of spark gaps and can provide much information on this topic.
The parallel bolts appear to work well, but many other schemes work as well or better. Using just the brackets in the manner described in Circuit Construction is easier, but with less adjustability of the gap. Another way is to use copper pipes mounted parallel, either two or an entire array. The traditional gap has rounded ends from which the sparks jump, and there may be different effects of the beam when they are produced in this manner as opposed to across a flat head bolt or parallel rods.
Rotary spark gaps use a motor to rotate multiple rods and when the rod passes through the gap, it sparks. This is a controllable method where the rate of the spark can be adjusted, and there is likely a lot of promise to this endeavor since the frequencies emitted would be more adjustable.
The distance of the spark gap in the device I made can vary from paper thin to almost 1/2". I usually use a gap of around 1/8". The larger the gap, the hotter the tube gets.