I use a Pipercross trumpet sock, the biggest one they do is 120mm wide I think. Looks like this but new ones dont thave the slot cut out of them like this old one...
You need to fit a piece of material or a chain drilled pipe like this inside it. If you dont it will get sucked into the connected pipe and compress, causing a restricted air flow...
Here is a photo of it fitted, that end is open uder there and its open on the sides by more than 60%. I made 275 bhp on a TD04 using this...