Ken,
I used a 3" pipe going into a flange, i have done a good bit of research on this forum and found the majority use 3" but don't hesitate to experiment and let us know how it works out.
I pushed the pipe through the hole and cut a shoulder in the hole 3/8" deep so I could glue a 3/8" piece of coupling to it then used silicone to hold it in the hole.
To summarize my setup, base is 3/4" plywood then the plenum is cut from 3/4" mdf, sealed by painting (some use different ways to seal), then cut the grid which included a hole large enough for the 2" pipe and a shoulder 3/8" below the grid but not into the plywood and cut a piece of 2" coupling to glue onto the 2" pipe to hold the pipe from dropping out of the hole.
Joe
2005 PRT Alpha 48x96
2013 Colombo 3hp spindle
Indexer (converted lathe)
Aspire 9.0