I had to really hunt to find my old favorite. It was cobbled together without plans,
10 or 12 years ago. It uses a standard grill igniter, with a wire epoxied to the side
and bent into a spark plug like look. It's glued into one cap of the
Y. The barrel
is about 4' of 2" and the chamber is a 4" Y. Since I had to open the chamber for every
shot, I made a couple custom tools to take off the caps.

Notice that the cap with igniter is screwed in backward, putting the spark closer to
center, but requiring a special tool for removal. (1" steel tube, with a rod handle.)