A reliable igniter for use with a combustion cannon, is a BBQ grill igniter coupled to two opposing sharpened bolts that almost meet inside the chamber (picture below). The gap can be as large as 1/4". The bolts were sharpened to help the spark make it across the gap. The BBQ igniters were available at both Orchard Supply Hardware and Home Depot in my area for around $10.00. The igniter comes with nuts, so it is just a matter of drilling the appropriate sized hole in the handle, inserting igniter and bolting it securely in place. I mounted the igniter on the 1.5" section of the "Y" for easy firing.

The spark jumps between the two bolts: