The little gap included with the #6 is all fine and good, but you will get bigger sparks with a rod gap. The parts are easy to come by at any hardware store. You need two 1 foot lengths of copper tubing, six street elbows, two endcaps, and two 45 degree elbows. Arrange them as shown in the figure and glue them together, allowing for electrical contact, as usual. You can save a couple of bucks by foregoing the street elbows. Just place the endcaps on the ends of the pipes, but corona leakage will rob you of about a quarter inch of spark.
To use the big gap, remove the caps from the collector tees and slip the 45 degree elbows over the 1 inch pipes (z). Adjust the distance between the rods to about 1 inch and run the Dirod. Little by little, nudge the gap wider and wider until no spark leaps. Then move it back in for the biggest spark possible.