I have the CO2 bubble directly into the intake of my HOB filter, I'm sure it's not 100% efficient, but it seems to work okay.
I pulled some medical tubing from an IV out of the garbage can when the nurses weren't looking last time my wife was in the hospital. The OD is about 1/8", it's just the right size that I can fit it through the hole on the cover of my AC filter where the nub sticks out for adjusting the flow, I've rubberbanded the tube to the intake pipe so that bubbles get sucked into the impeller that way instead of going into the tank. I don't know if it's any better but at least this way I only hear small bubbles hitting the impeller, instead of hearing larger bubbles from 1/4" tube hit the intake strainer then the impeller. I kind of feel like more small bubbles going into the impeller is more efficient than fewer larger bubbles.