Take care when sawing up the box, use gloves cause the SS-wrap around easily cuts into fingers etc.

There are several layers to cut through, bend the lid open and secure it with a piece of wood .
It is then possible to open up all small 3mm holes to approx. 8 mm diam. When you have
the box open You will see what I mean with restricted outlets.
Just follow the way the flow has to go from inlet....

When the internal job is done close lid and secure it with 1,5-2,5 mm thick Alu-stripes
( mine was 35 mm broad). I had the strips pre-drilled to take a lot of selfwinding small SS-screws to fully secure and close the box at all 3 cut lines.
There is really no need to apply "exhaust-gunk" to make it 100% tight but I had it at hand
and smeared the strips. I just drilled and screwed along the 2 sides first, then cutting the long stripe to length and finally secured that one. The alteration to the system will not be noticed visualy when the system is mounted on the car.