I have made a router table fence with a "dust extraction" feature:
As you can see there are gaps that would make the "box" not air tight. I need to use some kind of foam to fill those gaps.
The gaps are around 2-3 mm (5/64 - 1/8 inches) wide.
It doesn't need to be pretty, there isn't much mechanical stress, obviously it doesn't need to be food safe.
What solution/foam/etc would you recommend?
Later edit: Maybe even duct tape or something similar?

