Replacement cover for Hunter Plastics S450 Air Admittance Valve

This is a replacement cover for a Hunter Plastics (http://www.hunterplastics.co.uk) S450 Air Admittance valve (110mm) Yes, I really thought my first original upload to this site would be something interesting and beautiful, but needs must!

We have one of these valves installed in a downstairs bathroom. I needed to replace the cover (see pics) so knocked this up in 123D Design. It's my first attempt at 3D modelling and I'm certain someone can improve on my work.

As far as I am aware t