This is my intake. No it's not a toilet pipe flange, although I found the base materials in the same aisle
It's 3" pipe glued to a 4" block off plug with a hole cut in the middle (I think). Should have taken pics of the process. It is bolted to the plenum with silicone for air tightness.
ABS is great to work with, very light, dissolves in acetone and dries solid. I've repaired broken tabs from side covers by molding them, pouring melted ABS then letting it dry, still holding strong. Seems to be the same material as the plenum/airbox?
Anyway, this seems to work well, although a bit noisy when whacking the throttle open.