I didn't like the idea of using anything like a diffuser on a DIY 2L bottle. Just for the reason that if the diffuser clogged enough. The back pressure could blow the 2L bottle up.
But, with that said. You could probably put the CO2 line below the AC. And it would push the bubbles back down.
But, with that said. You could probably put the CO2 line below the AC. And it would push the bubbles back down.