Bob, I am not aware of anyone selling the cover already modified but that doesn't mean there isn't someone out there. It's very easy to do and I've seen several different ways to do it, any of which will work fine. I'll try to take a pic of mine and post it tonight.

Edited to add:
In addition to the way shown in this picture, I've seen guys solder the existing hole shut and drill and tap the center of the plate and screw a small brass fitting in like those used for muffler pressure. I put a hose on mine and run it inside the fuselage.