This is a blanking plate for use with sem-flex ducting.  This fits in the plenum or manifold to stop air leakage.  It was designed to fit the Zehnder manifolds or plenums and is composed of a cap which has slots for the Zehnder securing clips, and a gasket for reducing the air leakage.  

I printed the end cap in Sunlu PETG and the gasket with Fiberlogy FiberFlex 30D TPE so its nice and flexible, then used some silicone grease for fitting.  I have kept the tolerance as close to the inside diameter of the spigot to reduce the chance of leakage.