I just spend an ABSURD amount of time researching this. I landed on ASC IsoThermal TubeTraps. They have built-in diffusion (or absorption)!
https://www.acousticsciences.com/products/isothermal-tubetrap
- Extended bass absorption below 250 Hz
- Treble diffusion (or absorption) above 250 Hz
https://www.acousticsciences.com/products/isothermal-tubetrap