Since the pressures are so low, I duct taped the 1/8" holes and tried saw slits from a hacksaw, just as a test part. Just barely break through slots about 3/16" long at the bottom. I cut a dozen or so and it worked well, but very high volume of heat. The shape of the exit seems to play a large role. If I cut slots every 1/2" on a 3 ft pipe it would be too much flow. I'll play with a pattern.