Egregoros

Apropos to nothing: Compact 2 stroke engine exhaust expander ("resonator")

1. Exhaust from the engine
2. Expands in cone
3. Reverses direction and continues expanding around the cone
4. Contracts around the second cone
5. Reverses direction and contracts more inside of the cone
6. Exhaust exits

Picture 2: A normal expander.

I'd have a low resistance mesh to complete your catalytic process if that's the intent.

No catalyst, the 2 stroke expander is just a hollow tube, but the resonance of that shape is a big part of what makes the 2 stroke engine make power.