Sealing a clean out tee?

[southbalto]

When putting in our MetalFab Class A chimney last year I was suprised to see that the removable cup at the bottom is secured to the chimney with just a couple self tappnig screws. It seems like it makes for a poor seal.

I'm not haveing any major issues with draft, but I was wondering if it's a best practice to seal the cup....maybe with unfaced insulation or rockwool?

This is the cap:

 

[begreen]

I have seen better seals on DuraVent for example. Still, if it ain't broke ...