For spark ignition methods, I would recommend using an already existing product if possible. You can modify the length of the high voltage wire(s) and spark points into something more attractive as needed.
Often there is only one wire and the chassis is used for the return. This would be favorable in your design, since one wire is less clutter than two. They make some good small diameter high voltage wire, that you could probably chase up the inside of one of your pipes. Just make sure the voltage rating of the wire is higher than the voltage required for the spark gap. The dielectric strength of air is 3MV/m, so for 15kv wire the spark gap needs to be less than 5mm.
If you make your own spark points you will want them to be out of high temperature oxide resistant metal, or over time (100+ ignitions) the surfaces of the sparking materials will erode to the point of failure.
A pizeo igniter generates a high voltage spark by a push button spring striking a pizeo crystal. This obviously requires mechanical input, but wires can be extended to where you need them and no batteries are required.
An electric bbq igniter uses a AA or AAA battery, some internal electronics to create a pulse through a transformer to get the high voltage. Beyond the unit, the high voltage wire and ceramic insulated spark points are the same.
Another option to consider is a nicrome hot wire (especially if your device turns on and off frequently or has variable flow rates). You run low voltage DC electricity through the hot wire till it glows red and it will ignite the gas. Looks like someone on instructables used it for a similar application. This requires access to more electricity than the other options, but it makes your controls simpler since you would only need to modulate the gas. It is silent and doesn't make spark noises. It would also keep low gas flows ignited beyond the point of stable flame.
You would only need 5 mm of 20gauge nicrome and 5 volt at a couple amps would probably do the trick. You may need to put it just to the side of the gas stream so it doesn't cool down too much in a rapid on/off situation.
Pretty cool project, good luck as you move forward on it!